Add rake tasks for migration pages deployments between local and object storage

We need to reimplement https://gitlab.com/gitlab-org/gitlab/blob/master/lib/tasks/gitlab/artifacts/migrate.rake#L12 for pages deployments instead of artifacts.

These rake tasks will allow people to migrate to object storage if they have pages deployments stored locally. Or to migrate back, if they run pages on the single server or prefer to use NFS