muc: Ignore non-MUC (like error) messages

Noticed that Slack sends error messages back in response to chat state
notifications.

We're not doing anything with them currently and they bloat the sessionStorage

src/converse-muc.js

@@ -1161,7 +1161,7 @@
                     );
                     this.message_handler = _converse.connection.addHandler(
                         this.handleMUCMessage.bind(this),
-                        null, 'message', null, null, room_jid,
+                        null, 'message', 'groupchat', null, room_jid,
                         {'matchBareFromJid': true}
                     );
                 },