Fix stuck invites

If rejecting a remote invite fails with an error response don't fail
the entire request; instead mark the invite as locally rejected.

This fixes the bug where users can get stuck invites which they can
neither accept nor reject.

synapse/storage/roommember.py

@@ -102,6 +102,25 @@ class RoomMemberStore(SQLBaseStore):
                         event.state_key,
                     ))
 
+    @defer.inlineCallbacks
+    def locally_reject_invite(self, user_id, room_id):
+        sql = (
+            "UPDATE local_invites SET stream_id = ?, locally_rejected = ? WHERE"
+            " room_id = ? AND invitee = ? AND locally_rejected is NULL"
+            " AND replaced_by is NULL"
+        )
+
+        def f(txn, stream_ordering):
+            txn.execute(sql, (
+                stream_ordering,
+                True,
+                room_id,
+                user_id,
+            ))
+
+        with self._stream_id_gen.get_next() as stream_ordering:
+            yield self.runInteraction("locally_reject_invite", f, stream_ordering)
+
     def get_room_member(self, user_id, room_id):
         """Retrieve the current state of a room member.