Move mailer config to nucssa-core

[JJPro]

https://github.com/NUCSSA/nucssa-pickup/blob/master/lib/inc/mail-service/MailService.php#L9-L19

class MailService {
  public static function sendGridRelay($mailer) {
    $mailer->isSMTP();
    $mailer->Host = SENDGRID_SMTP_HOST;
    $mailer->SMTPAuth = true;
    $mailer->Port = SENDGRID_SMTP_PORT;
    $mailer->Username = SENDGRID_SMTP_USERNAME;
    $mailer->Password = SENDGRID_API_KEY;
    $mailer->SMTPSecure = 'TLS';
    $mailer->From = '[email protected]';
    $mailer->FromName = 'NUCSSA';
  }
 ...
}