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.

Clients disconnecting before we finish processing the request happens from time
to time. We don't need to yell about it

This reuses the same scheme as federation sender sharding

* Fix client reader sharding tests

* Newsfile

* Fix typing

* Update changelog.d/7853.misc

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

* Move mocking of http_client to tests

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

The Delete Room admin API allows server admins to remove rooms from server
and block these rooms.
`DELETE /_synapse/admin/v1/rooms/<room_id>`
It is a combination and improvement of "[Shutdown room](https://github.com/matrix-org/synapse/blob/develop/docs/admin_api/shutdown_room.md)" and "[Purge room](https://github.com/matrix-org/synapse/blob/develop/docs/admin_api/purge_room.md)" API.

Fixes: #6425 

It also fixes a bug in [synapse/storage/data_stores/main/room.py](synapse/storage/data_stores/main/room.py) in ` get_room_with_stats`.
It should return `None` if the room is unknown. But it returns an `IndexError`.
https://github.com/matrix-org/synapse/blob/901b1fa561e3cc661d78aa96d59802cf2078cb0d/synapse/storage/data_stores/main/room.py#L99-L105

Related to:
- #5575
- https://github.com/Awesome-Technologies/synapse-admin/issues/17



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

... instead of duplicating `config.signing_key[0]` everywhere

This table is no longer used, so we may as well stop populating it. Removing it
would prevent people rolling back to older releases of Synapse, so that can
happen in a future release.

* Fix spec compliance; tweaks without values are valid

(default to True, which is only concretely specified for
`highlight`, but it seems only reasonable to generalise)

* Changelog for 7766.

* Add documentation to `tweaks_for_actions`

May as well tidy up when I'm here.

* Add a test for `tweaks_for_actions`

The CI appears to use the latest version of isort, which is a problem when isort gets a major version bump. Rather than try to pin the version, I've done the necessary to make isort5 happy with synapse.

fixes #7016