Use `BatchModelLoader` for GroupType
As outlined in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10999#note_172098317, we should be using BatchModelLoader for the parent field in GroupType for performance reasons.
As outlined in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10999#note_172098317, we should be using BatchModelLoader for the parent field in GroupType for performance reasons.