Geo: Handle rename/move/delete of Pages

This is analogous to the repository syncing problem.

It's like we have legacy storage for Pages. Is there a security issue, now that Pages access control is a thing? There might be, I'm not sure. Access control is a little confusing.

Even if there isn't a major security issue, are there race conditions, like there were for repositories?

Should we go straight to hashed storage for Pages?

In https://gitlab.com/gitlab-org/gitlab-ee/issues/4611#note_132180830, @nick.thomas notes that the

API and serve-from-artifact approach

resolves this problem in a completely different way.

Edit: If we go that route, then ~Geo doesn't need to sync anything, and we don't need to do anything in this issue.