Use a background migration for migrating issues.closed_at

What does this MR do?

This re-implements the migrations rolled back in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16021 using background migrations. See the commit(s) for more details.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together