Finalize has_issues backfilling for external issue trackers
What does this MR do and why?
This MR finalizes BackfillHasIssuesForExternalIssueLinks batched background migration
Related to #442575 (closed)
Database review
The batched background migration was merged in %16.10
The batched background migration has finished running on GitLab.com
Edited by Michał Zając