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/DiscussionRenamedPost.js

@@ -18,6 +18,6 @@ export default class DiscussionRenamedPost extends EventPost {
     const oldTitle = post.content()[0];
     const newTitle = post.content()[1];
 
-    return ['changed the title from ', m('strong.old-title', oldTitle), ' to ', m('strong.new-title', newTitle), '.'];
+    return ['changed the title from ', m('strong.DiscussionRenamedPost-old', oldTitle), ' to ', m('strong.DiscussionRenamedPost-new', newTitle), '.'];
   }
 }