Drastically improve how the composer looks and behaves

- New, cleaner, more prominent look
- Make it statically positioned down the bottom on mobile, so you can
still scroll up to look at posts
- Fix some bugs with animation, jumping between views

js/forum/src/components/composer-body.js

@@ -21,8 +21,8 @@ export default class ComposerBody extends Component {
     });
   }
 
-  view() {
-    return m('div', {config: this.element}, [
+  view(className) {
+    return m('div', {className, config: this.element}, [
       avatar(this.props.user, {className: 'composer-avatar'}),
       m('div.composer-body', [
         m('ul.composer-header', listItems(this.headerItems().toArray())),