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/settings-page.js

@@ -19,6 +19,7 @@ export default class SettingsPage extends UserPage {
 
     this.setupUser(app.session.user());
     app.setTitle('Settings');
+    app.drawer.hide();
   }
 
   content() {