Ensure that post/page stubs created for nav menu items in WP4.7 can be edited

[westonruter]

Intercept/override the behavior of insertAutoDraftPost in nav menus to instead call the logic in the posts component instead. As such the nav_menus_created_posts setting won't even be used.

See https://core.trac.wordpress.org/ticket/34923

For the reverse, see #156.

[westonruter]

See integration point in has_published_pages method in #254