Creating theme customization require file

From this file, we will add all our customizations and remove them from
the core. This way, we can keep our theme HTML patterns and keep core changes
to a minimum.

  * Moving `core/templates/submenu.inc.php` breadcrumb into customizations `wpd_breadcrumb()`
  * Created a #wpd-buggenie body singleton to prepare upcoming global WPD theme overrides