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

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

Idea from matrix-org/synapse-dinsic#49

If there are *no* files with CRLF line endings, then the xargs exits with a
non-zero exit code (as expected), but then, since that is the last thing to
happen in the script, the script as a whole exits non-zero, making the whole
thing fail.

using `if/then/fi` instead of `&& (...)` means that the script exits with a
zero exit code.

Fixes #7901.

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

Synapse 1.18.0rc2 (2020-07-28)
==============================

Bugfixes
--------

- Fix an `AssertionError` exception introduced in v1.18.0rc1. ([\#7876](https://github.com/matrix-org/synapse/issues/7876))
- Fix experimental support for moving typing off master when worker is restarted, which is broken in v1.18.0rc1. ([\#7967](https://github.com/matrix-org/synapse/issues/7967))

Internal Changes
----------------

- Further optimise queueing of inbound replication commands. ([\#7876](https://github.com/matrix-org/synapse/issues/7876))

IIRC this doesn't break tests because its only hit on reconnection, or something.

Basically, when a process needs to fetch missing updates for the `typing` stream it needs to query the writer instance via HTTP (as we don't write typing notifications to the DB), the problem was that the endpoint (`streams`) was only registered on master and specifically not on the typing writer worker. 

This PRs adds a script to check for unix-line terminators in the repo. It will be used to address https://github.com/matrix-org/synapse/issues/7943 by adding the check to CI.

I've changed the original script slightly as proposed in https://github.com/matrix-org/pipelines/pull/81#discussion_r460580664

Most of the stuff we do for replication commands can be done synchronously. There's no point spinning up background processes if we're not going to need them.

Handling of incoming typing stream updates from replication was not
hooked up on master, effecting set ups where typing was handled on a
different worker.

This is really only a problem if the master process is also handling
sync requests, which is unlikely for those that are at the stage of
moving typing off.

The other observable effect is that if a worker restarts or a
replication connect drops then the typing worker will issue a
`POSITION typing`, triggering master process to try and stream *all*
typing updates from position 0.

Fixes #7907

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

If we send out an event which refers to `prev_events` which other servers in
the federation are missing, then (after a round or two of backfill attempts),
they will end up asking us for `/state_ids` at a particular point in the DAG.

As per https://github.com/matrix-org/synapse/issues/7893, this is quite
expensive, and we tend to see lots of very similar requests around the same
time.

We can therefore handle this much more efficiently by using a cache, which (a)
ensures that if we see the same request from multiple servers (or even the same
server, multiple times), then they share the result, and (b) any other servers
that miss the initial excitement can also benefit from the work.

[It's interesting to note that `/state` has a cache for exactly this
reason. `/state` is now essentially unused and replaced with `/state_ids`, but
evidently when we replaced it we forgot to add a cache to the new endpoint.]