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

src/Forum/ForumServiceProvider.php

@@ -74,6 +74,12 @@ class ForumServiceProvider extends ServiceProvider
             $this->action('Flarum\Forum\Actions\IndexAction')
         );
 
+        $routes->get(
+            '/notifications',
+            'flarum.forum.notifications',
+            $this->action('Flarum\Forum\Actions\IndexAction')
+        );
+
         $routes->get(
             '/logout',
             'flarum.forum.logout',