Checks if a user is registered
        :param course: a Course object
        :param username: The username of the user that we want to check. If None, uses self.session_username()
        :return: True if the user is registered, False else

def course_is_user_registered(self, course, username=None):
        """
        Checks if a user is registered
        :param course: a Course object
        :param username: The username of the user that we want to check. If None, uses self.session_username()
        :return: True if the user is registered, False else
        """
        if username is None:
            username = self.session_username()

        if self.has_staff_rights_on_course(course, username):
            return True

        return self._database.aggregations.find_one({"students": username, "courseid": course.get_id()}) is not None