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/forum/src/components/HeaderPrimary.js

@@ -9,7 +9,7 @@ import listItems from 'flarum/helpers/listItems';
 export default class HeaderPrimary extends Component {
   view() {
     return (
-      <ul className="header-controls">
+      <ul className="Header-controls">
         {listItems(this.items().toArray())}
       </ul>
     );