Enqueue backfilling of Epic resource_label_events

What does this MR do and why?

Implements the background migration to backfill resource_label_events for epic work items as described in Implement and enqueue backfilling of `resource_... (#454444 - closed) • Tetiana Chupryna • 18.11

This is part of the epic work items migration effort to transition from legacy epics to work items. The migration updates resource_label_events rows that currently point to legacy epics to instead point to their corresponding work item IDs.

Related Issues

• issue Implement and enqueue backfilling of `resource_... (#454444 - closed)
• Related to Backfill `resource_label_events` for epic work ... (&12747 - closed) (parent epic)
• Related to Migrate Label event API to present the resource... (!227491 - merged) (API presentation layer changes)