Consistently use room_id from federation request body (#8776)

* Consistently use room_id from federation request body

Some federation APIs have a redundant `room_id` path param (see
https://github.com/matrix-org/matrix-doc/issues/2330). We should make sure we
consistently use either the path param or the body param, and the body param is
easier.

* Kill off some references to "context"

Once upon a time, "rooms" were known as "contexts". I think this kills of the
last references to "contexts".

tests/handlers/test_federation.py

@@ -59,7 +59,6 @@ class FederationTestCase(unittest.HomeserverTestCase):
         )
 
         d = self.handler.on_exchange_third_party_invite_request(
-            room_id=room_id,
             event_dict={
                 "type": EventTypes.Member,
                 "room_id": room_id,