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/ModalManager.js

@@ -9,7 +9,7 @@ import Modal from 'flarum/components/Modal';
 export default class ModalManager extends Component {
   view() {
     return (
-      <div className="modal">
+      <div className="ModalManager modal fade">
         {this.component && this.component.render()}
       </div>
     );