We already had this the other way around (merge_request_diffs.merge_request_id),
but this is needed to gather only the most recent diffs for a set of merge
requests.

Fix out of sync with KubernetesService. Remove namespace pramas from controller. Use time_with_zone in schema. Remove Gcp::Clusters from safe_model_attributes.ym

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12932 we
freed up some names. But we were still renaming them.

This uses the existing ForkedProjectLinks

When no fork network exists for the source projects, we create a new
one with the correct source

The fork network will keep track of the root project as long as it's
present.