Major CSS revamp

- Get rid of Bootstrap (except we still rely on some JS)
- Use BEM class names
- Rework variables/theme config
- Fix various bugs, including some on mobile

The CSS is still not ideal – it needs to be cleaned up some more. But
that can be a focus for after beta.

js/lib/components/Separator.js

@@ -5,7 +5,7 @@ import Component from 'flarum/Component';
  */
 class Separator extends Component {
   view() {
-    return <li className="divider"/>;
+    return <li className="Dropdown-separator"/>;
   }
 }