Responsive fixes

- Make composer absolutely positioned on mobile to work around iOS 8/9
bug
- Make search results dropdown appear on mobile

closes flarum/core#137

js/forum/src/components/Search.js

@@ -75,7 +75,7 @@ export default class Search extends Component {
     }
 
     return (
-      <div className={'Search Dropdown ' + classList({
+      <div className={'Search ' + classList({
         open: this.value() && this.hasFocus,
         active: !!currentSearch,
         loading: !!this.loadingSources