[UX] Allow the admin bar to be shown/hidden via a toggle

https://www.drupal.org/project/admin_menu_dropdown is worth a look as far as extending upon the current admin_menu pulled into backdrop core UX that's going on. It's a very simple project that gives the (configurable) option of hiding the admin menu when pressing `. This is huge when demoing to low-tech / excessively visual clients who get off-put by seeing what's under the hood.