Migrate uploads rake task

What does this MR do?

Note: this MR is currently based on !3867 (merged)

Add the new gitlab:uploads:migrate[uploader_class, model_class, mounted_as] task.

This task is used to migrate uploads to a different object store.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

This is needed for GCP Migration, for https://gitlab.com/gitlab-com/infrastructure/issues/3575

Sample invocations

$ BATCH=1 RAILS_ENV=development bundle exec rake "gitlab:uploads:migrate[AvatarUploader, Project, :avatar]"
Migrated 1/1 files.
Migrated 1/1 files.

 === Migration summary ===
Migrated 2/2 files.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Follow-up of !3867 (merged) Related to #4163 (closed)