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.
git-svn-id: https://develop.svn.wordpress.org/trunk@38436 602fd350-edb4-49c9-b593-d223f7449a82
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