Geo: Bring back "events behind" text in Geo admin page

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5648 reverts the "events behind" design from https://gitlab.com/gitlab-org/gitlab-ee/issues/5146#note_62837301 because it was not displaying the right information.

I think we still want this, but to do this right we need to:

1. Subtract latest event ID - cursor event ID and show that rather than show cursor event ID
2. If it's negative (which we've seen before in https://gitlab.com/gitlab-org/gitlab-ee/issues/4850), perhaps link to an explanation why (https://gitlab.com/gitlab-org/gitlab-ee/issues/4850#note_58067468)?

While we are working on this, can we also display latest event ID on the primary.

/cc: @kushalpandya