code: def get_video_json(video):
      
  
      return json.dumps({
          'id': video.id,
          'edit_link': reverse('wagtailvideos:edit', args=(video.id,)),
          'title': video.title,
          'preview': {
              'url': video.thumbnail.url if video.thumbnail else '',
          }
      })
docs: helper function: given an image, return the json to pass back to the
      image chooser panel