Revoke access to forks when a user loses access to the upstream project
Related to the discussion in https://gitlab.com/gitlab-org/gitlab-ce/issues/43272#note_67933273
When organizations use the forking workflow on GitLab.com, they currently do not have a way to revoke access of a user to their own fork once they have revoked their access to the upstream project.
We would also need to prevent the user from Unlinking their fork from the upstream project. Unlinking is currently a step in destroying a project, so we would need to detach those from each other so a user is allowed to destroy their fork, but not unlink it.