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/TerminalPost.js

@@ -20,8 +20,8 @@ export default class TerminalPost extends Component {
 
     return (
       <span>
-        {username(user)}
-        {lastPost ? 'replied' : 'started'}
+        {username(user)}{' '}
+        {lastPost ? 'replied ' : 'started '}
         {humanTime(time)}
       </span>
     );