It is possible to select a Term's descendent as its parent

[mdawaffe]

When editing a hierarchical term, the dropdown selector for the term's parent has options for each of the term's descendants making it easy to create loops in the term hierarchy.

Attached uses the exclude_tree argument in wp_dropdown_category().

Attached depends on #14446.

Setting milestone to 3.0.2. If a user creates a category loop (for example), those categories in the loop disappear from view at wp-admin/edit-tags.php?taxonomy=category.

This is a frontend/web interface change. #14662 is a more comprehensive backend/API change.

[ryan]

(In [15798]) Don't show a term's descendants in the parent selector. Props mdawaffe. fixes #14663