Add ability to display the user id as a user picker hint for differentiation

Thanks to XWIKI-20937 it's possible to user a user's property as a hint. However, it's not possible to use the user id right now since there's no property for that.

Ideas for solutions:

• Idea 1: Add the user id as a computed xproperty in XWiki.XWikiUsers xclass (using $doc.documentReference and removing the wiki part)
• Idea 2: Modify NormalUserPreferencesConfigurationSource to handle a special userid property (or some other name) by overriding getProperty().