REST API: Include the sidebar ID when saving a widget.

This ensures backwards compatibility for code expecting this field to be present.

Props strategio, noisysocks, spacedmonkey.
See #53452.

git-svn-id: https://develop.svn.wordpress.org/trunk@51239 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-widgets-controller.php

@@ -478,6 +478,7 @@ class WP_REST_Widgets_Controller extends WP_REST_Controller {
 				"widget-$id_base" => array(
 					$number => $instance,
 				),
+				'sidebar'         => $sidebar_id,
 			);
 		} elseif ( isset( $request['form_data'] ) ) {
 			$form_data = $request['form_data'];