'Prefetches the value of the course tags for the specified users
for the specified course_id.
Args:
users: iterator of User objects
course_id: course identifier (CourseKey)
Returns:
course_tags: a dict of dicts,
where the primary key is the user\'s id
and the secondary key is the course tag\'s key'

@classmethod
def prefetch(cls, course_id, users):

    course_tags = defaultdict(dict)
    for tag in UserCourseTag.objects.filter(user__in=users, course_id=course_id).select_related('user__id'):
        course_tags[tag.user.id][tag.key] = tag.value
    get_cache(cls.CACHE_NAMESPACE)[cls._cache_key(course_id)] = course_tags
