Add `:throttled` as the urgency for BackgroundMigrations
As discussed in gitlab-org/gitlab#208810 (comment 302673975) we should mark the BackgroundMigrations
as urgency :throttled
.
In the first step they can be picked up by our catchall
fleet. But if necessary this can easily be changed in the chef-repo.
When running on Kubernetes, these could go in a separate pod that automatically gets spun up when there are things to be picked up.