"Community" support

- has been removed from mautrix/facebook in v0.3.3:
  31cac6fb5e

- has been removed from mautrix/signal in v0.2.2:
  1f27a608a6

- will be removed in the next mautrix/instagram release:
  e2ae1ca503

- will be removed in the next mautrix/twitter release:
  3893075265

Reverts b1b4ba501fdfaa, 90c9801c560b6, a3c84f78ca9c65a, ..

I haven't really traced it (yet), but on some servers, I'm observing
`ansible-playbook ... --tags=start` completing very slowly, waiting
to stop services. I can't reproduce this on all Matrix servers I manage.
I suspect that either the systemd version is to blame or that some
specific service is not responding well to some `docker kill/rm` command.

`ExecStop` seems to work great in all cases and it's what we've been
using for a very long time, so I'm reverting to that.

ExecStopPost should allow us to clean up (docker kill + docker rm)
even if the ExecStart (docker run ..) command failed, and not just after
a graceful service stop was initiated.

Source: https://www.freedesktop.org/software/systemd/man/systemd.service.html#ExecStopPost=

.. as per the default bridge config.

Source: f848710c01/mautrix_twitter/example-config.yamll (L147)