Automatically migrate legacy uploads

The following discussion from gitlab-ce!29409 should be addressed:

...snip...

Could you please open a follow up issue to add back the post-deploy migration after we've validated the rake task successfully on staging and production?

  • Run and verify rake gitlab:uploads:legacy:migrate on staging.gitlab.com
  • Fix any issues?
  • Run and verify rake gitlab:uploads:legacy:migrate on gitlab.com
  • Fix any issues?
  • Add post-deploy migration that runs the rake task

This will allow us to forget about legacy uploads, and get rid of AttachmentUploader.

Edited by Douglas Barbosa Alexandre