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

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.

Additionally we're delegating missing method calls on GpgKeySubkey to
GpgKey since most of the info required when verifying a signature is
found on GpgKey which is the parent of GpgKeySubkey

Conflicts:
	db/schema.rb

This is used in EE for the storage migration, and we want to use this
in CE as well to be able to migrate projects to hashed_storage.

This is a composite index on (target_project_id, merge_commit_sha, id)
that allows queries such as the following to use a full backwards index
scan:

    SELECT "merge_requests".*
    FROM "merge_requests"
    WHERE "merge_requests"."deleted_at" IS NULL
    AND "merge_requests"."target_project_id" = 13083
    AND "merge_requests"."merge_commit_sha" = 'e80a893f'
    ORDER BY "merge_requests"."id" DESC
    LIMIT 1;

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/38507