Make it a bit clearer what's going on.

Fixes https://github.com/matrix-org/synapse/issues/5431

`jinja2` was being imported even when it wasn't strictly necessary. This made it required to run Synapse, even if the functionality that required it wasn't enabled. This was causing new Synapse installations to crash on startup.

Email modules are now required.

There is a README.txt which always sets off this warning, which is a bit
alarming when you first start synapse. I don't think we need to warn about
this.

Fix /messages on worker when no token supplied

Support pagination API in client_reader worker

E_TOO_MANY_NEGATIVES

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

Add metrics for length of new extremities persisted.

Handle the case of `get_missing_events` failing

Add experimental option to reduce extremities.

Include xmlsec in the docker image

Fix typo in deactivation background job

Improve advice regarding poor performance

I had to add quite a lot of logging to diagnose a problem with 3pid
invites - we only logged the one failure which isn't all that
informative.

NB. I'm not convinced the logic of this loop is right: I think it
should just accept a single valid signature from a trusted source
rather than fail if *any* signature is invalid. Also it should
probably not skip the rest of middle loop if a check fails? However,
I'm deliberately not changing the logic here.

Fixes #5467.

Adds new config option `cleanup_extremities_with_dummy_events` which
periodically sends dummy events to rooms with more than 10 extremities.

THIS IS REALLY EXPERIMENTAL.

* Fix seven contrib files with Python syntax errors

Signed-off-by: cclauss <cclauss@me.com>

Add third party rules hook for 3PID invites

Add third party rules hook into create room