This should fix #7610.

Fixes https://github.com/matrix-org/synapse/issues/2431

Adds config option `encryption_enabled_by_default_for_room_type`, which determines whether encryption should be enabled with the default encryption algorithm in private or public rooms upon creation. Whether the room is private or public is decided based upon the room creation preset that is used.

Part of this PR is also pulling out all of the individual instances of `m.megolm.v1.aes-sha2` into a constant variable to eliminate typos ala https://github.com/matrix-org/synapse/pull/7637

Based on #7637

This reverts commit 8587b042.

* Ensure account data stream IDs are unique.

The account data stream is shared between three tables, and the maximum
allocated ID was tracked in a dedicated table. Updating the max ID
happened outside the transaction that allocated the ID, leading to a
race where if the server was restarted then the same ID could be
allocated but the max ID failed to be updated, leading it to be reused.

The ID generators have support for tracking across multiple tables, so
we may as well use that instead of a dedicated table.

* Fix bug in account data replication stream.

If the same stream ID was used in both global and room account data then
the getting updates for the replication stream would fail due to
`heapq.merge(..)` trying to compare a `str` with a `None`. (This is
because you'd have two rows like `(534, '!room')` and `(534, None)` from
the room and global account data tables).

Fix is just to order by stream ID, since we don't rely on the ordering
beyond that. The bug where stream IDs can be reused should be fixed now,
so this case shouldn't happen going forward.

Fixes #7617

Fixes https://github.com/matrix-org/synapse/issues/3177

* Clarify how to authenticate
* path params are not the same thing as query params
* Fix documentation for `/_synapse/admin/v2/users/<user_id>`

These things don't return Deferreds.

Allow new users to be registered via the admin API even if the monthly active user limit has been reached (#7263)

- Admin is able to
  - change displaynames
  - delete devices
  - list devices
  - get device informations

Fixes #7330

Closes #4382

Signed-off-by: Maxim Plotnikov <wgh@torlan.ru>

While working on https://github.com/matrix-org/synapse/issues/5665 I found myself digging into the `Ratelimiter` class and seeing that it was both:

* Rather undocumented, and
* causing a *lot* of config checks

This PR attempts to refactor and comment the `Ratelimiter` class, as well as encourage config file accesses to only be done at instantiation. 

Best to be reviewed commit-by-commit.

@uhoreg has confirmed these were both typos. They are only in comments and tests though, rather than anything critical.

Introduced in:

* https://github.com/matrix-org/synapse/pull/7157
* https://github.com/matrix-org/synapse/pull/5726

docs, default configs, comments. Nothing very significant.

Based on #7619 

async's `get_user_id_by_threepid` and its call stack.

Calls `self.get_success` on all deferred methods instead of abusing `self.pump()`. This has the benefit of working with coroutines, as well as checking that method execution completed successfully.

There are also a few small cleanups that I made in the process.

This is so the user is warned about the username not being valid as soon as possible, rather than only once they've finished UIA.

We already caught some exceptions, but not all.

Upstream have merged our changes
(https://github.com/spotify/dh-virtualenv/pull/300), so let's switch back to it
instead of using our fork.

* Expose `return_html_error`, and allow it to take a Jinja2 template instead of a raw string

* Clean up exception handling in SAML2ResponseResource

  * use the existing code in `return_html_error` instead of re-implementing it
    (giving it a jinja2 template rather than inventing a new form of template)

  * do the exception-catching in the REST layer rather than in the handler
    layer, to make sure we catch all exceptions.

Just in case people head straight to the troubleshooting section and find themselves at a dead end.

Synapse supports Python 3.8. We've been using it in CI for a while now.

It looks like `user_device_resync` was ignoring cross-signing keys from the results received from the remote server. This patch fixes this, by processing these keys using the same process `_handle_signing_key_updates` does (and effectively factor that part out of that function).