Introduce relative UserReference

Sometimes, it's useful to refer to a user in the context of the current wiki. When using entity references, it's possible. XWiki.USERNAME will refer to the USERNAME user in the current wiki.

It is currently not possible to use a UserReference for this. DocumentUserReference can be used to create a user reference from an entity reference, but this entity reference must be a DocumentReference, which requires a wiki to be set in the reference.

This means that APIs dealing with local users cannot currently use the UserReference and must use LocalDocumentReference or EntityReference instead, which works but is weaker.

A LocalDocumentUserReference or an EntityUserRference taking a relative reference would be nice to have.