Add label to back button, change behaviour

The back button longer shows if the user hasn't actually navigated anywhere. e.g. if they come in directly to a discussion, it will be hidden.

js/forum/src/components/DiscussionPage.js

@@ -159,6 +159,7 @@ export default class DiscussionPage extends Page {
   show(discussion) {
     this.discussion = discussion;
 
+    app.history.push('discussion', discussion.title());
     app.setTitle(discussion.title());
     app.setTitleCount(0);
 
@@ -273,7 +274,7 @@ export default class DiscussionPage extends Page {
     m.route(url, true);
     window.history.replaceState(null, document.title, url);
 
-    app.history.push('discussion');
+    app.history.push('discussion', discussion.title());
 
     // If the user hasn't read past here before, then we'll update their read
     // state and redraw.

js/forum/src/components/IndexPage.js

@@ -54,7 +54,7 @@ export default class IndexPage extends Page {
       app.cache.discussionList = new DiscussionList({params});
     }
 
-    app.history.push('index');
+    app.history.push('index', app.translator.trans('core.forum.header.discussions_button'));
 
     this.bodyClass = 'App--index';
   }

js/forum/src/components/UserPage.js

@@ -74,6 +74,7 @@ export default class UserPage extends Page {
   show(user) {
     this.user = user;
 
+    app.history.push('user', user.username());
     app.setTitle(user.username());
 
     m.redraw();