Customize: Allow users to more seamlessly create page-based nav menus during customization.
Introduces the ability to create stubs for the various post types to add to a given menu. This eliminates the need to leave the customizer to first create the post in the admin and then return to managing menus. Only the title of the newly-created post can be supplied; the post content will be blank and will need to be provided in the normal edit post screen outside the customizer, unless a plugin enables a post editing in the customizer experience. When a post is created and added to a nav menu in the customizer, the newly created post that is added to a menu is given the auto-draft status, and if the changes are not published, the auto-draft post will be automatically deleted within 7 days via wp_delete_auto_drafts(). However, if the customizer changes are saved, then these nav menu item auto-draft post stubs will be transitioned to publish.
Includes portions of code from the Customize Posts <https://github.com/xwp/wp-customize-posts> and Front-end Editor <https://github.com/iseulde/wp-front-end-editor> plugins.
For more information, see https://make.wordpress.org/core/2016/06/16/feature-proposal-content-authorship-in-menus-with-live-preview/
Props celloexpressions, westonruter, valendesigns, afercia, melchoyce, mapk, iseulde, mrahmadawais.
Fixes #34923.