Improve information provided in username reminder mail

When hitting the link "Forgot your username?" in the login screen, the sent mail provides the full user reference, eg "xwiki:XWiki.johndoe", rather than the reference name, eg "johndoe".

While technically the full reference will work on authentication, the user would more expect to receive the last part of it, since it corresponds to the identifier that they entered on registration or that was communicated to them.