[MOD] Do NOT JUST RANDOMLY BRIDGE PLUMBED MUCS

See README.MD for further thoughts that shall eventually be implemented.
Still need to deal with MUC/room "consent". PM'ing the MUC owner with a
form/using commands may be a nice course of action for that.

src/Routes/UserAck.c

@@ -85,6 +85,18 @@ RouteHead(RouteRoomAck, arr, argp)
         );
         goto end;
     }
+    if ((chatid = ParseeGetFromMUCID(args->data, muc)))
+    {
+        /* TODO: Should we map the plumbed MUC? */
+        Free(chatid);
+        chatid = NULL;
+        HttpResponseStatus(args->ctx, HTTP_METHOD_NOT_ALLOWED);
+        response = MatrixCreateError(
+            "M_UNRECOGNIZED",
+            "Room is already plumbed"
+        );
+        goto end;
+    }
     creator = StrConcat(
         4, 
         "@", args->data->config->sender_localpart, ":",