I18N: Add ability to change user's locale back to site's locale.

Previously there was no way to remove the user locale setting again, even though that might be desirable.

This adds a new 'Site Default' option to the user-specific language setting by introducing a new `show_site_locale_default` argument to `wp_dropdown_languages()`.

Props ocean90.
See #29783.
Fixes #38632.
Built from https://develop.svn.wordpress.org/trunk@39169


git-svn-id: http://core.svn.wordpress.org/trunk@39109 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php

@@ -1074,7 +1074,7 @@ class WP_REST_Users_Controller extends WP_REST_Controller {
 				'locale'    => array(
 					'description' => __( 'Locale for the resource.' ),
 					'type'        => 'string',
-					'enum'        => array_merge( array( 'en_US' ), get_available_languages() ),
+					'enum'        => array_merge( array( '', 'en_US' ), get_available_languages() ),
 					'context'     => array( 'edit' ),
 				),
 				'nickname'    => array(