Insert delta of current_state_events to be more efficient

Derive current_state_events from state groups

fix doc for purge_media_cache

purge_media_cache takes its arg from a query-param, not the POST body, for some
reason.

Make worker listener config backwards compat

Added username and password for turn server

It makes it possible to use a turn server that needs a username and
password instead of a token.

Update LruCache size estimate on clear

Allow configuring the Riot URL used in notification emails