Only consider webapp categories in App API (bug 963129)

https://bugzilla.mozilla.org/show_bug.cgi?id=963129

I initially removed that `queryset` parameter because specifying it did an extra useless query, but it turns out it's not longer the case and it's really useful for `PUT`/`PATCH`/`POST` if we have the same slug for different categories of different types. And we do.