get image's master url

    ...

    :param request: http GET request /renderer/master/url/<image_id>/
    :param image_id: the master image primary key
    :return: master url in a json dictionary

def get_master_url(request, image_id):
    '''
    get image's master url

    ...

    :param request: http GET request /renderer/master/url/<image_id>/
    :param image_id: the master image primary key
    :return: master url in a json dictionary
    '''
    im = get_object_or_404(MasterImage, pk=image_id)

    return JsonResponse({'url': im.get_master_url()})