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

Fix up preview URL API. Add tests.

This includes:

- Splitting out methods of a class into stand alone functions, to make
  them easier to test.
- Adding unit tests to split out functions, testing HTML -> preview.
- Handle the fact that elements in lxml may have tail text.

Limit number of extremeties in backfill request

Refactor user_delete_access_tokens. Invalidate get_user_by_access_token to slaves.

This works around a bug where if we make a backfill request with too
many extremeties it causes the request URI to be too long.

Merge branch 'fix_integrity_retry' of https://github.com/Ralith/synapse into Ralith-fix_integrity_retry