Adds categories admin section.

Adds the categories section to the Admin.

– Only allows operations when you're authenticated and authorized
– Lists categories
– Allows creation of a new category
– Edit and delete categories

I still need to add feedback to the user, but that will be a subsequent PR.
I'm working on a PR that moves the API things to a separate module. At that point in time, this will be abstracted to use that `value`. For now, to avoid a circular dependency on the `rave` module I've hardcoded it.