Add a cleanup worker to remove degenerated spaces

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 ?