https://gitlab.com/gitlab-org/gitlab/-/merge_requests/30313 fixed one
500 error when `application_settings.restricted_visibility_levels` is
NULL, but https://gitlab.com/gitlab-org/gitlab/-/merge_requests/22522
introduced the public visiblity check in two places.

To fix this problem and reduce code duplication, move this check into
`Gitlab::VisibilityLevel`, where NULL values of
`restricted_visibility_levels` was already handled.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/215638