Use Spring's method injection to set the dependencies for the message service

/**
  * Convenience method to allow us to change the configuration more easily. TODO Ideally, we
  * would be using Spring's method injection to set the dependencies for the message service.
  *
  * @return the ServiceContext
  */
private static MessageSender getMessageSender() {
  return new MailMessageSender(getMailSession());
 }