Make AppserviceHandler stream events from database

This is for two reasons:

1. Suppresses duplicates correctly, as the notifier doesn't do any
   duplicate suppression.
2. Makes it easier to connect the AppserviceHandler to the replication
   stream.

Add a media repository worker

Make notify_interested_services faster

Fix push_display_name_rename schema update

Set `Content-Security-Policy` on media repo

This is to inform browsers that they should sandbox the returned
media. This is particularly cruical for javascript/HTML files.

Move display name rule

Clean up _ServiceQueuer

Remove dead appservice code

Measure notify_interested_services

Don't update caches replication stream if tokens haven't advanced

To port existing rule actions & enable entries to the new name