Fixes https://github.com/matrix-org/synapse/issues/6583

I think this would have caught all the cases in
https://github.com/matrix-org/synapse/issues/7642 - and I think a 500 makes
more sense here than a 403

`StatsHandler` handles updates to the `current_state_delta_stream`, and updates room stats such as the amount of state events, joined users, etc.

However, it counts every new join membership as a new user entering a room (and that user being in another room), whereas it's possible for a user's membership status to go from join -> join, for instance when they change their per-room profile information.

This PR adds a check for join->join membership transitions, and bails out early, as none of the further checks are necessary at that point.

Due to this bug, membership stats in many rooms have ended up being wildly larger than their true values. I am not sure if we also want to include a migration step which recalculates these statistics (possibly using the `_populate_stats_process_rooms` bg update).

Bug introduced in the initial implementation https://github.com/matrix-org/synapse/pull/4338.

Add option ```purge``` to ```POST /_synapse/admin/v1/rooms/<room_id>/delete```
Fixes: #3761

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>

Fixes #7901.

Signed-off-by: Niklas Tittjung <nik_t.01@web.de>

Converts tests/rest/admin/test_room.py to have unix file endings after they were accidentally changed in #7613.

Keeping the same changelog as #7613 as it hasn't gone out in a release yet.

Fixes previews of Twitter URLs by using their oEmbed endpoint to grab content.