fix(conference): react to local role change only when it changes

We initialise the UI for isModerator = false on startup, so we should
not react to the event unless it gets out of sync.

conference.js

         room.on(ConferenceEvents.USER_ROLE_CHANGED, (id, role) => {
             if (this.isLocalId(id)) {
                 console.info(`My role changed, new role: ${role}`);
-                this.isModerator = room.isModerator();
-                APP.UI.updateLocalRole(room.isModerator());
+                if (this.isModerator !== room.isModerator()) {
+                    this.isModerator = room.isModerator();
+                    APP.UI.updateLocalRole(room.isModerator());
+                }
             } else {
                 let user = room.getParticipantById(id);
                 if (user) {