Don't pop conversations with timer change to top (#2084)

* Don't re-sort conversation list after expiration timer change

Now that we respond to the expiration timer included in contact and
group sync messages, we need to ensure that this doesn't pop
conversations to the top of the list.

* Introduce explaining variable for updateLastMessage filter

js/models/conversations.js

@@ -675,7 +675,9 @@
         return collection.fetchConversation(this.id, 1).then(function() {
             var lastMessage = collection.at(0);
             if (lastMessage) {
-                if (lastMessage.get('type') === 'verified-change') {
+                var type = lastMessage.get('type');
+                var shouldSkipUpdate = type === 'verified-change' || lastMessage.get('expirationTimerUpdate');
+                if (shouldSkipUpdate) {
                     return;
                 }
                 this.set({