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/routes/index.js

@@ -0,0 +1,7 @@
+import Ember from 'ember';
+
+export default Ember.Route.extend({
+  deactivate: function() {
+    this.controllerFor('application').set('backButtonTarget', null);
+  }
+});