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/ReplyComposer.js

@@ -27,7 +27,7 @@ export default class ReplyComposer extends ComposerBody {
 
     items.add('title', (
       <h3>
-        {icon('reply')} <a href={app.route.discussion(discussion)} config={m.route}>{discussion.title()}</a>
+        {icon('reply')}{' '}<a href={app.route.discussion(discussion)} config={m.route}>{discussion.title()}</a>
       </h3>
     ));