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.
REGISTRAR: fixed search for similar users

- Check if users existed was done through UES ID, which
  was not set (and is not stored in app) so now we search
  by ext source name and login.
Merge pull request #176 from zlamalp/app

GUI, REGISTRAR: better check for similar users