Significantly improve mobile UX

Most of #137 done.

- Use FastClick to make everything feel more responsive
- Use transforms for animations to make them silky smooth
- Style the drawer the same as the header to keep things simple
- Revert to fixed composer, but allow it to be minimised
- Add a separate notifications page for mobile so it’s easy to go back
- Add indicator to the menu button when there are unread notifications
- Close the drawer when navigating away
- Make dropdowns/modals scrollable
- Many other mobile tweaks and bug fixes

Didn’t take much care to keep CSS clean, due to #103

js/forum/src/components/post-loading.js

@@ -4,7 +4,7 @@ import avatar from 'flarum/helpers/avatar';
 export default class PostLoadingComponent extends Component {
   view() {
     return m('div.post.comment-post.loading-post.fake-post',
-      m('header.post-header', avatar(), m('div.fake-text')),
+      m('header.post-header', avatar(), ' ', m('div.fake-text')),
       m('div.post-body', m('div.fake-text'), m('div.fake-text'), m('div.fake-text'))
     );
   }