* Add infrastructure to the presence handler to track sync requests in external processes

* Expire stale entries for dead external processes

* Add an http endpoint for making users as syncing

Add some docstrings and comments.

* Fixes

Split out the auth handler

Refactor SyncHandler

special case m.room.third_party_invite event auth to match invites,

Use user_id in email greeting if display name is null

special case m.room.third_party_invite event auth to match invites, otherwise they get out of sync and you get https://github.com/vector-im/vector-web/issues/1208

Ensure we always return boolean in push rules

Cache get_event_reference_hashes

Fix room list spidering

Add secondary_directory_servers option to fetch room list from other servers

Limit number of notifications in an email notification

Otherwise it tries to start the remote public room list updating looping call which breaks.

Poll for updates from remote servers, waiting for the poll if there's no cache entry.

Add vector.im to default secondary_directory_servers and add comment explaining it's not a permanent solution

Fix GET /push_rules

Add caches to bulk_get_push_rules*

Move the AS handler out of the Handlers object.

Access it directly from the homeserver itself. It already wasn't
inheriting from BaseHandler storing it on the Handlers object was
already somewhat dubious.

Split out the room list handler

So I can use it from federation bits without pulling in all the handlers.