Serialize local user/group references when saving rights to subjects on the same wiki as the page on which rights are saved

Currently, when subjects of security rules are saved (users or groups), they're serialized as absolute references.
They should be serialized as local references, relative to the page on which they're set (not relative to the current context) as the rights UI does when rights are set from the UI.

However, only the wiki part should be missing from the reference should be the wiki, the reference should not be a full relative reference.