Finds a direct mention (a mention that is at the beginning) in message text
            and returns the user ID which was mentioned. If there is no direct mention, returns None

def __parse_direct_mention(self, message_text):
        """
            Finds a direct mention (a mention that is at the beginning) in message text
            and returns the user ID which was mentioned. If there is no direct mention, returns None
        """
        matches = re.search(MENTION_REGEX, message_text)
        # the first group contains the username, the second group contains the remaining message
        return (matches.group(1), listify(matches.group(2).strip())) if matches else (None, None)