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/DiscussionComposer.js

@@ -37,7 +37,7 @@ export default class DiscussionComposer extends ComposerBody {
 
     items.add('title', (
       <h3>
-        <input className="form-control"
+        <input className="FormControl"
           value={this.title()}
           oninput={m.withAttr('value', this.title)}
           placeholder={this.props.titlePlaceholder}