… and refactor push rule code along the way.

Keep track of all prev_events since if we know that an event is a
prev_event it is referenced and does not qualify as a forward extremity.

It seems in order to test this I may also have to impl the email 3pid
route? I need to call the set_pusher route somehow.

TIL body.device_id != (*body).device_id, which is pretty bad, so I
renamed body.device_id to body.sender_device