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.
Migrated from D6599 (view on Phabricator)