Replace prepend_entity with prepend_if_ee helper (EE-port)

Replaces prepend_entity with prepend_if_ee for EE-specific parts of CE entities.

prepend_entity remains for now, so we have a chance to announce the change. It will be removed in a follow-up merge request.

closes https://gitlab.com/gitlab-org/gitlab-ee/issues/13639

CE-version: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32214