Generate a work item for every epic created

As part of the migration of epics to work items, we need to create an associated work-item of type epic for every legacy epic that is created. We should use issue_id column on epics table to hold the association. The work item created should have the same following fields as the legacy epic after its creation:

• group_id - called namespace_id at issues table
• title
• description
• author_id
• updated_by_id
• last_edited_by_id
• last_edited_at
• created_at
• updated_at
• closed_by_id
• closed_at
• state_id
• confidential

Implementation proposal - !134920 (closed)

1. When a legacy epic is created we open a transaction
2. A work item is created inside the transaction
3. Still in the transaction we take work item id and assign to legacy epic issue_id column
4. Save both objects

ps: If there is a failure when saving any of the objects then none should be created. The transaction should rollback.