DRY up registration event callbacks

This was just a special case of the extension.on/trigger interface.

js/views/inbox_view.js

@@ -42,7 +42,7 @@
                 }
             }.bind(this));
 
-            extension.onMessage('message', function(message) {
+            extension.on('message', function(message) {
                 this.conversations.fetch({id: message.conversationId}).then(function() {
                     this.conversations.get(message.conversationId).fetchMessages();
                 }.bind(this));