REGISTRAR: better pre-fill of user's name

- When filling form item mapped to perun attribute displayName,
  if value is not provided by perun or federation (concrete attribute),
  try to recreate it from parsed name (which is best of displayName,
  cn or givenName+sn).

  So if value is provided by Perun, it's filled.
  If it's provided by federation and present, it's overwritten.
  If it's provided by federation and missing, try to get best
  of all provided attributes (if not set by Perun already).