Simplify global back button

The behaviour is not overly intuitive, and the icon wasn't helping
(hamburger icon usually means "menu"). Now the back button always goes
back to the index, no matter where you are, and there's a tooltip that
says "Back to discussion list".

js/lib/components/Navigation.js

@@ -50,10 +50,10 @@ export default class Navigation extends Component {
     const previous = history.getPrevious() || {};
 
     return LinkButton.component({
-      className: 'Button Navigation-back ' + (previous.title ? '' : 'Button--icon'),
+      className: 'Button Navigation-back Button--icon',
       href: history.backUrl(),
       icon: 'chevron-left',
-      children: previous.title,
+      title: previous.title,
       config: () => {},
       onclick: e => {
         if (e.shiftKey || e.ctrlKey || e.metaKey || e.which === 2) return;