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/dropdown-split.js

@@ -21,7 +21,7 @@ export default class DropdownSplit extends Component {
 
     return m('div', {className: 'dropdown dropdown-split btn-group item-count-'+(items.length)+' '+this.props.className}, [
       ActionButton.component(buttonProps),
-      m('a[href=javascript:;]', {className: 'dropdown-toggle '+this.props.buttonClass, 'data-toggle': 'dropdown'}, [
+      m('a[href=javascript:;]', {className: 'dropdown-toggle btn-icon '+this.props.buttonClass, 'data-toggle': 'dropdown'}, [
         icon('caret-down icon-caret'),
         icon((this.props.icon || 'ellipsis-v')+' icon'),
       ]),