Revert "deposit: Remove 'parent' deposit"

This reverts commit f6905cdf.

That commit was a first step toward making loaders write releases instead of revisions.

Unfortunately, we will still write revisions for a non-negligeable time, so I prefer to defer the removal of parent deposit revisions to the moment we actually make that switch, so we don't end up with inconsistent revisions.

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Migrated from D6599 (view on Phabricator)