We should replace [`nested_menu_arrow.gif`](https://github.com/activeadmin/activeadmin/tree/master/app/assets/images/active_admin/nested_menu_arrow.gif) and [`nested_menu_arrow_dark.gif`](https://github.com/activeadmin/activeadmin/tree/master/app/assets/images/active_admin/nested_menu_arrow_dark.gif) with css arrows.  Maybe @cohart can work on this?