Missing validation on typing EDUs

[squahtx]

When we receive a typing EDU from a remote homeserver, we neglect to check that said homeserver is in the room the typing EDU is for.

There was a check that looked like it was doing the job, except it was really checking that the local homeserver is in the room a second time:

synapse/synapse/handlers/typing.py

Lines 365 to 369 in 1a209ef

	domains = await self._storage_controllers.state.get_current_hosts_in_room(
	room_id
	)
	if self.server_name in domains:

related: #11456