Remove usage of extension.trigger

This trigger function uses chrome's runtime message passing api, which
traverses between different windows in our runtime, but we only trigger
the updateInbox event from the backgroud page, so we don't need to use
that api, which requires some extra cpu/memory overhead.

// FREEBIE

js/panel_controller.js

@@ -41,15 +41,14 @@
         }
     });
 
-    function updateInbox() {
+    window.updateInbox = function() {
         conversations.fetchActive().then(function() {
             inbox.reset(conversations.filter(function(model) {
                 return model.get('active_at');
             }));
         });
-    }
+    };
 
-    extension.on('updateInbox', updateInbox);
     updateInbox();
     setUnreadCount(storage.get("unreadCount", 0));