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/application.js

@@ -14,7 +14,41 @@ export default Ember.Controller.extend({
   searchQuery: '',
   searchActive: false,
 
+  history: null,
+
+  init: function() {
+    this._super();
+    this.set('history', []);
+    this.pushHistory('index', '/');
+  },
+
+  pushHistory: function(name, url) {
+    var url = url || this.get('target.url');
+    var last = this.get('history').get('lastObject');
+    if (last && last.name === name) {
+      last.url = url;
+    } else {
+      this.get('history').pushObject({name: name, url: url});
+    }
+  },
+
+  popHistory: function(name) {
+    var last = this.get('history').get('lastObject');
+    if (last && last.name === name) {
+      this.get('history').popObject();
+    }
+  },
+
+  canGoBack: Ember.computed('history.length', function() {
+    return this.get('history.length') > 1;
+  }),
+
   actions: {
+    goBack: function() {
+      this.get('history').popObject();
+      var history = this.get('history').get('lastObject');
+      this.transitionToRoute.call(this, history.url);
+    },
     search: function(query) {
       this.transitionToRoute('index', {queryParams: {searchQuery: query, sort: query ? 'relevance' : 'recent'}});
     },

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');
     },