Send out invite rejections over federation

ensure that leave events sent via `send_leave` are sent on to the rest of the
federation.

Fixes #10222.

changelog.d/10223.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug which meant that invite rejections were not sent out over federation in a timely manner.

synapse/handlers/federation.py

@@ -1980,6 +1980,13 @@ async def on_send_leave_request(self, origin: str, pdu: EventBase) -> None:
 
         event.internal_metadata.outlier = False
 
+        # Send this event on behalf of the other server.
+        #
+        # The remote server isn't a full participant in the room at this point, so
+        # may not have an up-to-date list of the other homeservers participating in
+        # the room, so we send it on their behalf.
+        event.internal_metadata.send_on_behalf_of = origin
+
         context = await self.state_handler.compute_event_context(event)
         await self._auth_and_persist_event(origin, event, context)