Add support for event_id_only push format
Add support for event_id_only push format

Param in the data dict of a pusher that tells an HTTP pusher to
send just the event_id of the event it's notifying about and the
notification counts. For clients that want to go & fetch the body
of the event themselves anyway.

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