Add a confirmation step to the SSO login flow

See merge request new-vector/synapse!2

Fix #6910

Properly convert a coroutine into a Deferred in federation_base to fix an error when joining a room.

Some small fixes to the copy in #6907.

Make room alias lists peekable

Clean up and update federation docs

it's not in the spec yet, so needs to be unstable. Also add a feature flag for it. Also add a test for admin users.

we have hundreds of path_regexes (see #5118), so let's not convert the same
bytes to str for each of them.

* Increase DB/CPU perf of `_is_server_still_joined` check.

For rooms with large amount of state a single user leaving could cause
us to go and load a lot of membership events and then pull out
membership state in a large number of batches.

* Newsfile

* Update synapse/storage/persist_events.py

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Fix adding if too soon

* Update docstring

* Review comments

* Woops typo

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

Fix log in message retention purge jobs

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

As per
https://github.com/matrix-org/matrix-doc/pull/2432#pullrequestreview-360566830,
make room alias lists accessible to users outside world_readable rooms.

... and set it everywhere it's called.

while we're here, rename it for consistency with `check_user_in_room` (and to
help check that I haven't missed any instances)

these were getting a bit unwieldy, so let's combine `check_joined_room` and
`check_user_was_in_room` into a single `check_user_in_room`.

per matrix-org/matrix-doc#2432

Rewrite _EventInternalMetadata to back it with a dict

Add mention and warning about ACME v1 deprecation to the TLS config

We do this by moving the recursive query to be fully in the DB.

Update INSTALL.md to recommend reverse proxying and warn about ACMEv1 deprecation