Re-queue backfilling of work_item_dates_sources with epic dates data

What does this MR do and why?

Re-queue backfilling of work_item_dates_sources with epic dates data

Here, we are adding a migration which just calls the original migration where, for each existing epic, we need to have a work_item_dates_sources record associated to the epic's work item. These records store the epic dates data including the booleans start_date_is_fixed and due_date_is_fixed that should have a default value even if dates are not present.