Issues/222 category editing

Replaces the existing lib/public_body_categories.rb category logic with 2 database backed models - PublicBodyCategory (aliased to PublicBodyCategories for compatibility purposes) and PublicBodyHeading.

Initial data population can be achieved by running PublicBodyCategories.load_categories as before, and a new admin panel allows superusers to add and edit categories and headings and allocate categories to one or more headings.