Allow documenting why "Private" repositories aren't available

GNOME chose to disallow the creation of private repositories and forks to avoid its GitLab instance being used for non-GNOME related projects, and prevent CI abuse, amongst other things.

However this is a problem when trying to create a confidential merge request, as once forked, the end user won't be able to set their fork's visibility to Private, the drop-down menu item will just appear grayed out.

As this functionality is paramount to dealing with security issues, it would be useful for the reason why the functionality isn't available, either as a text field, or a ? bubble. In GNOME's case, it would likely say something like:

Private repositories are only available to handle security issues. Contact the sysadmin team to your repository private.