* Allow room creator to send MSC2716 related events in existing room versions

Discussed at https://github.com/matrix-org/matrix-doc/pull/2716/#discussion_r682474869

Restoring `get_create_event_for_room_txn` from,
https://github.com/matrix-org/synapse/pull/10245/commits/44bb3f0cf5cb365ef9281554daceeecfb17cc94d

* Add changelog

* Stop people from trying to redact MSC2716 events in unsupported room versions

* Populate rooms.creator column for easy lookup

> From some [out of band discussion](https://matrix.to/#/!UytJQHLQYfvYWsGrGY:jki.re/$p2fKESoFst038x6pOOmsY0C49S2gLKMr0jhNMz_JJz0?via=jki.re&via=matrix.org), my plan is to use `rooms.creator`. But currently, we don't fill in `creator` for remote rooms when a user is invited to a room for example. So we need to add some code to fill in `creator` wherever we add to the `rooms` table. And also add a background update to fill in the rows missing `creator` (we can use the same logic that `get_create_event_for_room_txn` is doing by looking in the state events to get the `creator`).
>
> https://github.com/matrix-org/synapse/pull/10566#issuecomment-901616642

* Remove and switch away from get_create_event_for_room_txn

* Fix no create event being found because no state events persisted yet

* Fix and add tests for rooms creator bg update

* Populate rooms.creator field for easy lookup

Part of https://github.com/matrix-org/synapse/pull/10566

 - Fill in creator whenever we insert into the rooms table
 - Add background update to backfill any missing creator values

* Add changelog

* Fix usage

* Remove extra delta already included in #10697

* Don't worry about setting creator for invite

* Only iterate over rows missing the creator

See https://github.com/matrix-org/synapse/pull/10697#discussion_r695940898

* Use constant to fetch room creator field

See https://github.com/matrix-org/synapse/pull/10697#discussion_r696803029

* More protection from other random types

See https://github.com/matrix-org/synapse/pull/10697#discussion_r696806853

* Move new background update to end of list

See https://github.com/matrix-org/synapse/pull/10697#discussion_r696814181

* Fix query casing

* Fix ambiguity iterating over cursor instead of list

Fix `psycopg2.ProgrammingError: no results to fetch` error
when tests run with Postgres.

```
SYNAPSE_POSTGRES=1 SYNAPSE_TEST_LOG_LEVEL=INFO python -m twisted.trial tests.storage.databases.main.test_room
```

---

We use `txn.fetchall` because it will return the results as a
list or an empty list when there are no results.

Docs:

> `cursor` objects are iterable, so, instead of calling explicitly fetchone() in a loop, the object itself can be used:
>
> https://www.psycopg.org/docs/cursor.html#cursor-iterable

And I'm guessing iterating over a raw cursor does something weird when there are no results.

---

Test CI failure: https://github.com/matrix-org/synapse/pull/10697/checks?check_run_id=3468916530
```
tests.test_visibility.FilterEventsForServerTestCase.test_large_room
===============================================================================
[FAIL]
Traceback (most recent call last):
  File "/home/runner/work/synapse/synapse/tests/storage/databases/main/test_room.py", line 85, in test_background_populate_rooms_creator_column
    self.get_success(
  File "/home/runner/work/synapse/synapse/tests/unittest.py", line 500, in get_success
    return self.successResultOf(d)
  File "/home/runner/work/synapse/synapse/.tox/py/lib/python3.9/site-packages/twisted/trial/_synctest.py", line 700, in successResultOf
    self.fail(
twisted.trial.unittest.FailTest: Success result expected on <Deferred at 0x7f4022f3eb50 current result: None>, found failure result instead:
Traceback (most recent call last):
  File "/home/runner/work/synapse/synapse/.tox/py/lib/python3.9/site-packages/twisted/internet/defer.py", line 701, in errback
    self._startRunCallbacks(fail)
  File "/home/runner/work/synapse/synapse/.tox/py/lib/python3.9/site-packages/twisted/internet/defer.py", line 764, in _startRunCallbacks
    self._runCallbacks()
  File "/home/runner/work/synapse/synapse/.tox/py/lib/python3.9/site-packages/twisted/internet/defer.py", line 858, in _runCallbacks
    current.result = callback(  # type: ignore[misc]
  File "/home/runner/work/synapse/synapse/.tox/py/lib/python3.9/site-packages/twisted/internet/defer.py", line 1751, in gotResult
    current_context.run(_inlineCallbacks, r, gen, status)
--- <exception caught here> ---
  File "/home/runner/work/synapse/synapse/.tox/py/lib/python3.9/site-packages/twisted/internet/defer.py", line 1657, in _inlineCallbacks
    result = current_context.run(
  File "/home/runner/work/synapse/synapse/.tox/py/lib/python3.9/site-packages/twisted/python/failure.py", line 500, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/home/runner/work/synapse/synapse/synapse/storage/background_updates.py", line 224, in do_next_background_update
    await self._do_background_update(desired_duration_ms)
  File "/home/runner/work/synapse/synapse/synapse/storage/background_updates.py", line 261, in _do_background_update
    items_updated = await update_handler(progress, batch_size)
  File "/home/runner/work/synapse/synapse/synapse/storage/databases/main/room.py", line 1399, in _background_populate_rooms_creator_column
    end = await self.db_pool.runInteraction(
  File "/home/runner/work/synapse/synapse/synapse/storage/database.py", line 686, in runInteraction
    result = await self.runWithConnection(
  File "/home/runner/work/synapse/synapse/synapse/storage/database.py", line 791, in runWithConnection
    return await make_deferred_yieldable(
  File "/home/runner/work/synapse/synapse/.tox/py/lib/python3.9/site-packages/twisted/internet/defer.py", line 858, in _runCallbacks
    current.result = callback(  # type: ignore[misc]
  File "/home/runner/work/synapse/synapse/tests/server.py", line 425, in <lambda>
    d.addCallback(lambda x: function(*args, **kwargs))
  File "/home/runner/work/synapse/synapse/.tox/py/lib/python3.9/site-packages/twisted/enterprise/adbapi.py", line 293, in _runWithConnection
    compat.reraise(excValue, excTraceback)
  File "/home/runner/work/synapse/synapse/.tox/py/lib/python3.9/site-packages/twisted/python/deprecate.py", line 298, in deprecatedFunction
    return function(*args, **kwargs)
  File "/home/runner/work/synapse/synapse/.tox/py/lib/python3.9/site-packages/twisted/python/compat.py", line 404, in reraise
    raise exception.with_traceback(traceback)
  File "/home/runner/work/synapse/synapse/.tox/py/lib/python3.9/site-packages/twisted/enterprise/adbapi.py", line 284, in _runWithConnection
    result = func(conn, *args, **kw)
  File "/home/runner/work/synapse/synapse/synapse/storage/database.py", line 786, in inner_func
    return func(db_conn, *args, **kwargs)
  File "/home/runner/work/synapse/synapse/synapse/storage/database.py", line 554, in new_transaction
    r = func(cursor, *args, **kwargs)
  File "/home/runner/work/synapse/synapse/synapse/storage/databases/main/room.py", line 1375, in _background_populate_rooms_creator_column_txn
    for room_id, event_json in txn:
psycopg2.ProgrammingError: no results to fetch
```

* Move code not under the MSC2716 room version underneath an experimental config option

See https://github.com/matrix-org/synapse/pull/10566#issuecomment-906437909

* Add ordering to rooms creator background update

See https://github.com/matrix-org/synapse/pull/10697#discussion_r696815277

* Add comment to better document constant

See https://github.com/matrix-org/synapse/pull/10697#discussion_r699674458

* Use constant field