Only create notifications for unread messages

In some cases, we have already received a read receipt for an incoming
message by the time we go to create a notification about it. In this
case, we should skip the notification.

// FREEBIE

js/models/messages.js

@@ -357,7 +357,9 @@
                         message.save().then(function() {
                             conversation.save().then(function() {
                                 conversation.trigger('newmessage', message);
-                                conversation.notify(message);
+                                if (message.get('unread')) {
+                                    conversation.notify(message);
+                                }
                                 resolve();
                             });
                         });