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/initializers/boot.js

@@ -31,7 +31,6 @@ export default function boot(app) {
   }
 
   app.routes[defaultAction].path = '/';
-  app.history.push(defaultAction, '/');
 
   m.startComputation();