WARNING: There are a lot of config breaking changes in this one, watch out!

* KEYCLOAK2* env vars have gone away, they were experimental.

* TELEGRAM_BRIDGE_ADMIN* is replaced by TELEGRAM_BRIDGE_PERMISSIONS.

* SIGNAL_BRIDGE_ADMIN* is replaced by SIGNAL_BRIDGE_PERMISSIONS.

* The homeserver config has been trimmed, see coop-cloud/matrix-synapse#33 for more.

* Bridge logging is only ERROR level now to minimise leaking plaintext.

* It is possible to use SSO & federation env vars in combination now.

* Media retention is now configurable with #MEDIA_RETENTION_* env vars.

@decentral1se