Notify users for events in rooms they join.
Notify users for events in rooms they join.

Change how the notifier updates the map from room_id to user streams on
receiving a join event. Make it update the map when it notifies for the
join event, rather than using the "user_joined_room" distributor signal

TribeMedia/synapse,TribeMedia/synapse,matrix-org/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse,TribeMedia/synapse