Adds admin menu icon support to CustomPost on MP6.

...by using 'content' CSS property, and counting on
the presence of the Dashicons font.
Hides 'parent' field on CustomTaxonomy, when uneeded
Removes icon script (now in core Atlas).
Bugfix: looping over null when term created directly

A term created outside of the admin interface was causing
a loop over empty POST variables.