Tidy public body category management UI

Make the forms for editing categories and category headings more uniform
with other admin pages

- Put the dangerous actions in a well
- Remove large well for returning to the index page
Fix incorrect count for heading children

Was initially counting the number of headings rather than the number
of categories inside each heading.
Add JS confirmation for deleting heading/category
Merge branch '1975-destroy-pbc-heading-msg' into rails-3-develop