'Retrieve verification deadlines for particular courses.
Arguments:
course_keys (list): List of `CourseKey`s.
Returns:
dict: Map of course keys to datetimes (verification deadlines)'

@classmethod
def deadlines_for_courses(cls, course_keys):

    all_deadlines = cache.get(cls.ALL_DEADLINES_CACHE_KEY)
    if (all_deadlines is None):
        all_deadlines = {deadline.course_key: deadline.deadline for deadline in VerificationDeadline.objects.all()}
        cache.set(cls.ALL_DEADLINES_CACHE_KEY, all_deadlines)
    return {course_key: all_deadlines[course_key] for course_key in course_keys if (course_key in all_deadlines)}
