Sync epics parent_id and relative position to work items

Epics table have parent_id and relative_position fields these are used to show an ordered list of child epics.

As part of the migration of epics to work items we need to migrate data from these columns into a new widget for work items which holds data at work_item_parent_links.

Proposal

1. Whenever a new parent is assigned to an epic we verify if both parent and child already have associated work items created. If not we skip the next steps.
2. Create a new record at work_item_parent_links by copying:
   • legacy_epic.parent.issue_id to work_item_parent_links.work_item_parent_id
   • legacy_epic.relative_position to work_item_parent_links.relative_position
   • legacy_epic.issue_id to work_item_parent_links.work_item_id
3. When a child epic is removed from its parent we destroy the same record at work_item_parent_links where:
   • work_item_parent_links.work_item_id is equal to child_epic.issue_id
   • work_item_parent_links.work_item_parent_id is equal to child_epic.parent.issue_id
4. Backfill work_item_parent_tables using background migration following the same logic from step 2.