Build permission name for model_class (like 'app.add_model').

def build_permission_name(model_class, prefix):
    """ Build permission name for model_class (like 'app.add_model'). """
    model_name = model_class._meta.object_name.lower()
    app_label = model_class._meta.app_label
    action_name = prefix
    perm = '%s.%s_%s' % (app_label, action_name, model_name)
    return perm