Add audit log messages when a forked project is unlinked from parent

Due to gitlab-com/gl-infra/production#2889 (closed), we switched gitlab-org/gitlab from a public project to an internal one.

Unfortunately this unlinked 2000+ forks from this parent, causing us to recover this data in gitlab-com/gl-infra/production#2890 (closed).

Every time a project is unlinked, we should have an audit or some other log message generated.