'return the auto-enrollable mode from given dict
Args:
modes_dict (dict): course modes.
Returns:
String: Mode name'

@classmethod
def auto_enroll_mode(cls, course_id, modes_dict=None):

    if (modes_dict is None):
        modes_dict = cls.modes_for_course_dict(course_id)
    if (cls.HONOR in modes_dict):
        return cls.HONOR
    elif (cls.AUDIT in modes_dict):
        return cls.AUDIT
