Geo: Verification of Job artifacts

Data integrity is essential to Geo. If the data is worth replicating, then it's worth verifying.

• Wait for https://gitlab.com/gitlab-org/gitlab-ee/issues/5591 Edit: The former issue was modified over time to only cover front-end. This issue actually depends on &1817 (closed), and maybe #8922 (closed)
• Verify Job artifacts as well

Refactor if needed, don't reimplement things.