Fetch messages after fetching members.

This is done to avoid unnecessary repaints and reflows (caused when a
message has already rendered and then an occupant is created and
attached to that message, cauring a re-render).

Related to #1266

src/headless/converse-muc.js

@@ -365,7 +365,6 @@ converse.plugins.add('converse-muc', {
                     this.removeNonMembers();
                     await this.refreshRoomFeatures();
                     this.clearMessages(); // XXX: This should be conditional
-                    this.fetchMessages();
                     if (!u.isPersistableModel(this)) {
                         // XXX: Happens during tests, nothing to do if this
                         // is a hanging chatbox (i.e. not in the collection anymore).