Improve composer appearance/usability on mobile

On mobile:
- Move submit button to right side of toolbar
- Move first header item to toolbar
- Size textarea correctly

js/forum/src/components/ComposerBody.js

@@ -56,7 +56,7 @@ export default class ComposerBody extends Component {
     this.editor.props.disabled = this.loading;
 
     return (
-      <div className="ComposerBody">
+      <div className={'ComposerBody ' + (this.props.className || '')}>
         {avatar(this.props.user, {className: 'ComposerBody-avatar'})}
         <div className="ComposerBody-content">
           <ul className="ComposerBody-header">{listItems(this.headerItems().toArray())}</ul>