Generate a URL for the archive of an entity..

        @param entity_id The ID of the entity to look up as a string
               or reference.
        @param channel Optional channel name.

def archive_url(self, entity_id, channel=None):
        '''Generate a URL for the archive of an entity..

        @param entity_id The ID of the entity to look up as a string
               or reference.
        @param channel Optional channel name.
        '''
        url = '{}/{}/archive'.format(self.url, _get_path(entity_id))
        return _add_channel(url, channel)