A theme can opt-in for tailored starter content to apply to the customizer when previewing the theme on a fresh install, when `fresh_site` is at its initial `1` value. Starter content is staged in the customizer and does not go live unless the changes are published. Initial starter content is added to Twenty Seventeen.
* The `fresh_site` flag is cleared when a published post or page is saved, when widgets are modified, or when the customizer state is saved.
* Starter content is registered via `starter-content` theme support, where the argument is an array containing `widgets`, `posts`, `nav_menus`, `options`, and `theme_mods`. Posts/pages in starter content are created with the `auto-draft` status, re-using the page/post stubs feature added to nav menus and the static front page controls.
* A `get_theme_starter_content` filter allows for plugins to extend a theme's starter content.
* Starter content in themes can/should re-use existing starter content items in core by using named placeholders.
* Import theme starter content into customized state when fresh site.
* Prevent original_title differences from causing refreshes if title is present.
* Ensure nav menu item url is set according to object when previewing.
* Make sure initial saved state is false if there are dirty settings without an existing changeset.
* Ensure dirty settings are cleaned upon changeset publishing.
Props helen, westonruter, ocean90.
Fixes#38114, #38533.
Built from https://develop.svn.wordpress.org/trunk@38991
git-svn-id: http://core.svn.wordpress.org/trunk@38934 1a063a9b-81f0-0310-95a4-ce76da25c4cd
