Remove the 'Category' model, use constants instead (bug 987880)

https://bugzilla.mozilla.org/show_bug.cgi?id=987880

- [x] Cleanups in forms & validation (though devhub is working fine)
- [x] Fixes for existing tests
- [x] Migration to convert existing data (for apps, collections, feed items)
*Edit:* actually, no, it's too fragile. Just editing the `default` keyword argument is not enough, we might end up with empty values in the database and it wouldn't handle that. We could have a `_categories` field and a `categories` `@property` wrapper around it but I'm not a big fan.