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/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();