'For a given `course_id`, return a list of `(filename, url)` tuples.
Calls the `url` method of the underlying storage backend. Returned
urls can be plugged straight into an href'

def links_for(self, course_id):

    course_dir = self.path_to(course_id)
    try:
        (_, filenames) = self.storage.listdir(course_dir)
    except OSError:
        return []
    files = [(filename, os.path.join(course_dir, filename)) for filename in filenames]
    files.sort(key=(lambda f: self.storage.modified_time(f[1])), reverse=True)
    return [(filename, self.storage.url(full_path)) for (filename, full_path) in files]
