* 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.
Merges [39411] to 4.7 branch.
See #38114.
Fixes#38928 for 4.7.
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