Uploads should honor the background object storage migration

The following discussion from !3867 (merged) should be addressed:

• @nick.thomas started a discussion: (+2 comments)

  I didn't see that this setting was honoured for uploads.

  For LFS objects we have https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/models/lfs_object.rb#L16 which, I think, is the code that schedules the background jobs. Do we need the same for uploads?