Fix race condition between migrate_at_uuid and migrate_relatedItems

For @pbauer 

migrate_relatedItems should be executed before migrate_at_uuid, because migrate_at_uuid should remove UID to prevent AT UID related events breaking unmigrated AT references

See discussion at https://github.com/plone/Products.contentmigration/commit/32b4639a7544d109304fee6e9327d682fc637cc5