Pillify room notifs in the timeline
This scans text nodes in the DOM for room notifications and turns
them into pills. Changes the pillification code around a bit so it
works with text nodes. Uses the push processor directly to test
the event against the room notifiation rule so we know whether
this event would actually trigger a room notification (needs to
hook into push at a lower level because otherwise our own room
notifications would not pillify since our own events never
generate notifications).
Requires https://github.com/matrix-org/matrix-js-sdk/pull/565
