Unread counts

Update unreadCounts per-conversation on incoming messages. Render unread
conversations with font-weigh: bold in the inbox view.

To ensure that the inbox and conversation views remain in sync, the
background page now ensures that the same models objects are used for
both views.

js/inbox_controller.js

@@ -29,9 +29,10 @@
         }
     });
 
+    inbox.on('change:active_at', inbox.sort);
+
     function fetch() {
         window.inbox.fetch({
-            reset: true,
             index: {
                 name: 'inbox', // 'inbox' index on active_at
                 order: 'desc'  // ORDER timestamp DESC