CORE-5832 Added DELETE /admin/apps/categories/:id
CORE-5832 Added POST /admin/apps/categories parent_id validation.
CORE-5832 Added metadactyl.conrad.admin-categories.

Refactored /admin/apps/categories functions into this new namespace.
CORE-5832 Added PATCH /admin/apps/categories/:id

Refactored some validation into functions in metadactyl.conrad.admin-categories.