This is a follow up of https://gitlab.com/gitlab-org/gitlab-ce/issues/51716 When a Project is destroyed, we'll have some orphaned records of `Cluster::KubernetesNamespaces`. We need to clean those through a cleanup worker. Also clean up `Cluster::KubernetesNamespaces` which should no longer be present for cases where the project transfers group. Open question: - Should we also remove namespace, service account and token from Cluster? - One reason to do this is because of security implications (since the token will exist on the Cluster side). - But also we disallow to create the same namespace, and the token is limited to the namespace (with `edit` role) so it poses minimal problems - This will destroy all records of deployments to the namespace, and also events. - What about when a project is transferred to another Group ?