Backfill RelatedWorkItems (automatically)

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While we can ensure that every related_epic_link has a work_item_links, we don't ensure this the other way around.

So when Relate quick actions doesn't sync back to legac... (#515885 - closed) we introduced a bug in the REST API only. While making the related-work-item links the SSoT is part of the solution - we still need to keep the IDs consistent in the REST API.

Until we figured out how to best approach this with #519265 we might want to automatically backfill data on the legacy tables when we figure out it's missing.

So we'd have

1. WorkItem tables to be the source of truth
2. LEFT JOIN the legacy tables
3. When records are missing, we create them