Customize: Prevent auto-draft post/page stubs from being saved with empty slugs or published with non-unique slugs.

* Allow `WP_Customize_Nav_Menus::insert_auto_draft_post()` to take full post array to pass to `wp_insert_post()`, except for `post_status`. Require `post_title`. 
* Ensure empty `post_name` gets explicitly set to slugified `post_title`.
* Explicitly allow only `post_type` and `post_title` params in `WP_Customize_Nav_Menus::ajax_insert_auto_draft_post()`.
* Use `wp_update_post()` instead of `wp_publish_post()` to ensure unique slugs are assigned to published auto-draft posts.
* Re-use `WP_Customize_Nav_Menus::insert_auto_draft_post()` when inserting stubs from starter content.
 
See #38114, #38013, #34923.
Fixes #38539.

Built from https://develop.svn.wordpress.org/trunk@39038


git-svn-id: http://core.svn.wordpress.org/trunk@38980 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.7-beta1-39037';
+$wp_version = '4.7-beta1-39038';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.