Adds categories/preferences admin section.

Adds the categories + preferences sections to the Admin.

#### Categories commit

– Only allows operations when you're authenticated and authorized
– Lists categories
– Allows creation of a new category
– Edit and delete categories

#### Preferences commit

– Removes `/preference` endpoint. We're treating the preferences as a single resource.
– Adds save functionality to preferences page.

I still need to add feedback to the user, but that will be a subsequent PR.