Add the ability to perform background migrations

What does this MR do?

This MR adds the ability to perform background migrations. See https://gitlab.com/gitlab-org/gitlab-ce/issues/32146 for more information.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests
  • Added for this feature/bug
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together