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/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.7-beta2-39168';
+$wp_version = '4.7-beta2-39169';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.