In some cases it is desirable to preserve particular users that are not
backed by any entity in the authoritative user directory.

For instance it may be useful to retain the `root` user created by
default on GitLab during the initial setup for programmatic access
from deployment tools like Helm and Ansible.

Or the `ghost` user that GitLab uses as a placeholder for deleted users.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>