GUI, REGISTRAR: better check for similar users

GUI
- Do not call check for similar users, if user is known on application.
  Call ended up ok, but it's not really necessary.

REGISTRAR
- Check for similar users now uses findUsers(searchString) which omits
  user's titles and can check for emails too.
- Order of checks used when user not found by UserExtSource is:
  PreferredMail, Mail, FullName, LastName.
- Added authorization for checkForSimilarUsers() method.