Finalize the backfilling of epic work item records and validate the not null constraint on epics.issue_id column

Every BBM needs a finalize migration that ensures the original migration completed.

So this issue is to track that work. This needs to:

• create a sync migration that would finalize the backfilling of work items
• add a migration that introduces a add_not_null_constraint :epics, :issue_id, validate: true