formatAtSign method
Returns null if atsign is null else the formatted atsign.
atsign must be non-null.
Implementation
String formatAtSign(String atsign) {
  atsign = atsign.trim().toLowerCase().replaceAll(' ', '');
  atsign = !atsign.startsWith('@') ? '@$atsign' : atsign;
  return atsign;
}