Customize: Reuse existing non-auto-draft posts and existing auto-draft posts...

Customize: Reuse existing non-auto-draft posts and existing auto-draft posts in the customized state with matching slugs when applying starter content.

* Updates `wp_unique_post_slug()` to ignore `auto-draft` posts. Prevents publishing multiple posts that have the same slugs from starter content.
* Fixes fatal error when attempting to save an header_image setting from a non-admin context.
* Fixes substituting attachment symbols in options and theme mods.
* Fixes applying starter content for header images and background images.

See #38114.
Fixes #38928.

Built from https://develop.svn.wordpress.org/trunk@39411


git-svn-id: https://core.svn.wordpress.org/trunk@39351 1a063a9b-81f0-0310-95a4-ce76da25c4cd