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

Joriks/demo python3

Configure the demo servers to use untrusted
tls certs so that they communicate with each other.

This configuration makes them very unsafe so I've added warnings about
it in the readme.

Handle HttpResponseException when using federation client.

Fix email notifications for unnamed rooms with multiple people

Only start background group attestation renewals on master

Including workers!

Fix 3PID invite room state over federation.

Allow server admins to define implementations of extra rules for allowing or denying incoming events

Prometheus histograms are cumalative

Fix background job for deactivated flag

Fixes that when a user exchanges a 3PID invite for a proper invite over
federation it does not include the `invite_room_state` key.

This was due to synapse incorrectly sending out two invite requests.