Bring back support classes for background migrations
A number of users (including me and others in https://gitlab.com/gitlab-org/gitlab-ee/issues/12976#note_194851224) noticed
uninitialized constant Gitlab::BackgroundMigration messages (e.g.
PopulateMergeRequestMetricsWithEventsDataImproved) in their Sidekiq logs, and I think this is because gitlab-ce@8469f59d786be6762908f62d642625790999cb9b removed these supporting classes (e.g.
@yorickpeterse Is there a reason we shouldn't just have these classes available in case Sidekiq jobs are still scheduled? We don't have a good mechanism right now to tell users, "Background migrations are still running, wait X until they are finished!" By the time they upgrade, it's too late.