Cannot fork public project if public visibility level is restricted

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/14692

If a GitLab install has the public visibility level restricted, but still have some public projects created by an administrator, users cannot fork the public project. They receive an error 'Fork error! Visibility level Public visibility has been restricted by your GitLab administrator'.

We should allow users to fork projects but choose another visibility level in this case.

cc/ @JobV @rspeicher Thoughts?