Remove restrictions on deletion of PublicBodyCategories and PublicBodyHeadings

Allows any `PublicBodyHeading` or `PublicBodyCategory` to be deleted via the admin interface, regardless of whether they are acting as containers for `PublicBodyCategory` or `PublicBody` objects (respectively).

Closes #1969
Looks like this is actually an okay pattern http://everydayrails.com/2012/04/07/testing-series-rspec-controllers.html