[WIP] Add Category Restrictions

This feature enhances the existing permissions system by introducing a category whiltelist.

This is a response to #143.

### TODO ###

- [x] Enhance Category Model + Migration
- [x] Add interface to manage whitelist
- [x] Enforce basic whitelist
- [ ] Expand whitelist to ensure cascaded (hierarchial) permission restrictions
- [x] Ensure blacklisted results don't appear in searches (may not be possible)

### Enhancements ###

* make management interface better?