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/lib/components/nav-item.js

@@ -4,7 +4,7 @@ import icon from 'flarum/helpers/icon'
 export default class NavItem extends Component {
   view() {
     var active = this.constructor.active(this.props);
-    return m('li'+(active ? '.active' : ''), m('a', {
+    return m('li'+(active ? '.active' : ''), m('a.has-icon', {
       href: this.props.href,
       onclick: this.props.onclick,
       config: m.route