code: def to_deprecated_list_repr(self):
          
          return ['c4x', self.org, self.course, self.block_type, self.block_id, None]
docs: Thumbnail locations are stored as lists [c4x, org, course, thumbnail, path, None] in contentstore.mongo
          That should be the only use of this method, but the method is general enough to provide the pre-opaque
          Location fields as an array in the old order with the tag.