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/Switch.js

@@ -8,10 +8,10 @@ export default class Switch extends Checkbox {
   static initProps(props) {
     super.initProps(props);
 
-    props.className += ' switch';
+    props.className = (props.className || '') + ' Checkbox--switch';
   }
 
   getDisplay() {
-    return '';
+    return this.loading ? super.getDisplay() : '';
   }
 }