Do not bundle aggregations for APIs which shouldn't include them. (#11592)

And make bundling aggregations opt-in, instead of opt-out to avoid
having APIs to include extraneous data (and being much heavier than
necessary).

synapse/handlers/message.py

@@ -246,7 +246,9 @@ class MessageHandler:
                 room_state = room_state_events[membership_event_id]
 
         now = self.clock.time_msec()
-        events = await self._event_serializer.serialize_events(room_state.values(), now)
+        events = await self._event_serializer.serialize_events(
+            room_state.values(), now, bundle_aggregations=True
+        )
         return events
 
     async def get_joined_members(self, requester: Requester, room_id: str) -> dict: