Draft: Support fork source projects going private

Supersedes Remove marketing site (!310)

We have configured the fork projects with prevent_destroy = true, so we need to support the case where the source project goes private.

In those cases, we can hardcode the last known name and path while archiving the fork. That way we don't have to remove forks of projects that go private. They stay at the last known public commit and can stay as a public reference.