`COUNT()` never returns `NULL`. A `COUNT(*)` over 0 rows is 0 and a
`COUNT(NULL)` is also 0.

A couple of safety-checks to hopefully stop people doing what I just did, and create a storage
function which only works the first time it is called (and not when it is re-run due to a database
concurrency error or similar). 

* Splits the logic for parsing HTML from the resource handling code.
* Fix a circular import in the oEmbed code (which uses the HTML parsing code).
* Renames some of the HTML parsing methods to:
  * Make it clear which methods are "internal" to the module.
  * Clarify what the methods do.

Make `get_device` return None if the device doesn't exist rather than raising an exception. (#11565)

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>

The event still needs to have `auth_events` defined to be valid.

Split out from https://github.com/matrix-org/synapse/pull/11114

Part of MSC2716: https://github.com/matrix-org/matrix-doc/pull/2716

We did some work on making sure the `state_groups` were shared in
https://github.com/matrix-org/synapse/pull/10975

C.f. https://github.com/matrix-org/synapse/pull/11494#pullrequestreview-827780886

Create a new dict helper method `simple_insert_many_values_txn`, which takes
raw row values, rather than {key=>value} dicts. This saves us a bunch of dict
munging, and makes it easier to use generators rather than creating
intermediate lists and dicts.

this field is never read, so we may as well stop populating it.

This should fix pushers with an email in non-canonical form is used as
the pushkey.

And expand some type hints in the receipts storage module.

Co-authored-by: David Robertson <davidr@element.io>

Somehow I'd managed to get my database in a pickle with stream ids. These
changes were useful to debug.

Pull the DataStore from the HomeServer instance, which
always exists.

This reverts commit a77c3698.

* Move sync_token up to the top
* Pull out _get_ignored_users
* Try to signpost the body of `_generate_sync_entry_for_rooms`
* Pull out _calculate_user_changes

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

Fixes #11517

After #10847, `looping_background_call` would print an error in the logs
every time a non-async function was called. Since the error would be
caught and ignored immediately, there were no other side effects.

If, for some reason, we don't have the create event, we should still be able to
purge a room.

Stabilise support for MSC2918 refresh tokens as they have now been merged into the Matrix specification. (#11435)

As a step towards allowing back-channel logout for OIDC.