Improve global back button. Goes back to previous interface.

It’s not quite like the browser’s back button because it doesn’t
necessarily go back to the last URL; rather, it goes back to the last
interface. So if you go into a discussion, then go to a different
discussion via the side pane, the back button will still take you back
to the index (not the previous discussion).

ember/app/controllers/index.js

@@ -32,10 +32,6 @@ export default Ember.Controller.extend(UseComposer, Paneable, {
   },
 
   actions: {
-    transitionFromBackButton: function() {
-      this.transitionToRoute('index');
-    },
-
     loadMore: function() {
       this.get('index').send('loadMore');
     },