Geo: Create an automated way of showing the differences with Geo checksum mismatches

Right now if the primary and secondary have a checksum verification failure, it's not obvious what exactly is the problem. We need to follow some manual steps https://gitlab.com/gitlab-org/gitlab-ee/issues/9228 to identify the differences between the references used to generate the checksum. It would be nice if Geo did this diff automatically and showed it in the admin screen.