Add a config option to change whether unread push notification counts are per-message or per-room (#8820)

This PR adds a new config option to the `push` section of the homeserver config, `group_unread_count_by_room`. By default Synapse will group push notifications by room (so if you have 1000 unread messages, if they lie in 55 rooms, you'll see an unread count on your phone of 55).

However, it is also useful to be able to send out the true count of unread messages if desired. If `group_unread_count_by_room` is set to `false`, then with the above example, one would see an unread count of 1000 (email anyone?).