Foreground verification of uploads and LFS objects

What does this MR do?

Reworks the gitlab:uploads:check rake task and introduces a gitlab:lfs:check rake task to match it.

The CE and EE implementations have gotten rather out of sync, so I also take the opportunity to rework things so they can be more closely aligned. See the associated EE MR for more details: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4753

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

This is just a starting point for verification, not the endpoint. For Geo and GitLab.com, we're also going to have a beautiful frontend with background verification, based on the excellent repository verification work recently demoed by @dbalexandre. This is effectively a stop-gap until that's complete.

CI job artifacts aren't included yet, as we don't store a checksum for them anywhere 😅 . I plan to address this in a follow-up MR which will make use of the framework provided in this one.

Why was this MR needed?

It's important to be able to verify that your user-uploaded files are accurate. They may become corrupted due to bitrot or some other error. Since we store checksums in the database, this is simple to do.

A simple foreground rake task for verification is often sufficient for smaller instances.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Related to gitlab-ee#4862