muc: render tab upon chatroom add

So that it shows unread messages upon page reload.

src/converse-muc.js

@@ -2388,6 +2388,7 @@
                     this.model.on('change:muc_domain', this.onDomainChange, this);
                     this.model.on('change:nick', this.onNickChange, this);
                     _converse.chatboxes.on('change:num_unread', this.renderTab, this);
+                    _converse.chatboxes.on('add', _.debounce(this.renderTab, 100), this);
                 },
 
                 render: function () {