Fix a 500 error resulting from empty room_ids

POST /_matrix/client/api/v1/rooms//send/a.b.c gave a 500 error, because we
assumed that rooms always had at least one character.

Pull out sender when computing search results

Implement rank function for SQLite FTS

Reject events which are too large

Support filtering events represented as dicts.

Add filters to search.

This is useful because the emphemeral events such as presence and
typing are represented as dicts inside synapse.

Refactor api.filtering to have a Filter API

Cap the time to retry txns to appservices to 8.5 minutes

SPEC-222

Add config option to disable password login

Add some unit tests of prune_events()

Include typing events in initial v2 sync

Conflicts:
	synapse/handlers/sync.py

Add rooms that the user has left under archived in v2 sync.

Don't 500 on /login when the email doesn't map to a valid user ID.

There's been numerous issues with people playing around with their
application service and then not receiving events from their HS for
ages due to backoff timers reaching crazy heights (albeit capped at
< 1 day).

Reduce the max time between pokes to be 8.5 minutes (2^9 secs) which
is quick enough for people to wait it out (avg wait time being 4.25 min)
but long enough to actually give the AS breathing room if it needs it.