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)
null is a bit special in the collection API (means "match everything") so I used `null=True` to allow NULLs for compatibility but you're right that it's confusing, it's generally not a very good thing for `CharFields`.