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/utils/History.js

@@ -93,6 +93,6 @@ export default class History {
   home() {
     this.stack.splice(1);
 
-    m.route(this.stack[0].url);
+    m.route('/');
   }
 }