When we do an invite rejection, save the signed leave event to the db

During a rejection of an invite received over federation, we ask a remote
server to make us a `leave` event, then sign it, then send that with
`send_leave`.

We were saving the *unsigned* version of the event (which has a different event
id to the signed version) to our db (and sending it to the clients), whereas
other servers in the room will have seen the *signed* version. We're not aware
of any actual problems that caused, except that it makes the database confusing
to look at and generally leaves the room in a weird state.

synapse/handlers/federation.py

@@ -1101,15 +1101,15 @@ class FederationHandler(BaseHandler):
                 user_id,
                 "leave"
             )
-            signed_event = self._sign_event(event)
+            event = self._sign_event(event)
         except SynapseError:
             raise
         except CodeMessageException as e:
             logger.warn("Failed to reject invite: %s", e)
             raise SynapseError(500, "Failed to reject invite")
 
-        # Try the host we successfully got a response to /make_join/
-        # request first.
+        # Try the host that we succesfully called /make_leave/ on first for
+        # the /send_leave/ request.
         try:
             target_hosts.remove(origin)
             target_hosts.insert(0, origin)
@@ -1119,7 +1119,7 @@ class FederationHandler(BaseHandler):
         try:
             yield self.replication_layer.send_leave(
                 target_hosts,
-                signed_event
+                event
             )
         except SynapseError:
             raise