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/index.js

@@ -1,8 +1,11 @@
 import Ember from 'ember';
 
 import AddCssClassToBody from 'flarum/mixins/add-css-class-to-body';
+import PushesHistory from 'flarum/mixins/pushes-history';
+
+export default Ember.Route.extend(AddCssClassToBody, PushesHistory, {
+  historyKey: 'index',
 
-export default Ember.Route.extend(AddCssClassToBody, {
   cachedModel: null,
 
   model: function() {
@@ -13,7 +16,7 @@ export default Ember.Route.extend(AddCssClassToBody, {
   },
 
   setupController: function(controller, model) {
-    controller.set('model', model);
+    this._super(controller, model);
 
     if (!model.get('length')) {
       controller.send('loadResults');