Customizer post/page stubs fail to get unique slugs when publishing

[westonruter]

I noticed that after initializing Twenty Seventeen all of the Top nav menu items all linked to the homepage. It turns out that the auto-draft status post/page stubs were not getting a post_name specified and so the permalink for them would point to the homepage.

Likewise, when adding new nav menu items, I'm able to supply titles which then result in pages/posts that have the same post_name as other items. This is due to wp_publish_posts() being used which bypasses the unique slug check.

See #38114, #38013, #34923

[westonruter]

Naturally this issue was addressed by \WP_Customize_Nav_Menus::insert_auto_draft_post() and since we require the nav menus component anyway for inserting post/page stubs, we might as well re-use it.

[westonruter]

#38568 was marked as a duplicate.

[westonruter]

​https://github.com/xwp/wordpress-develop/pull/190

[westonruter]

In 39038:

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.