Allow starter content to apply in a new theme when switching from another theme containing changes

[westonruter]

See #38114. At the moment, starter content is only applied to the customized state if if there is no changeset post existing for the current UUID. However, if I have a fresh site and I preview Twenty Seventeen and then, without activating, I do a theme switch to another theme that has starter content, I should see that theme's starter content merged on top of the previous starter content since the site is still fresh. A complication here is with the auto-draft post/page. The existence of the changeset provided a locking mechanism to prevent these posts from being created repeatedly in a given session. What needs to be done, however, is to ensure that they only get created once for a given theme's imported starter content.

[westonruter]

In 39241:

Customize: Allow starter content to apply in a new theme when switching from another theme containing changes.

• Ensure that starter content can apply from theme B after previewing starter content in theme A.
• Introduce new starter_content flag in changeset setting params which is used to capture whether a value is starter content and thus can be overridden.
• Create changeset up-front with starter_content flags instead of waiting for AUTOSAVE_INTERVAL.
• Eliminate instantiation of settings for widget instances in favor of directly calling sanitize_widget_js_instance. This eliminates issues with looking for widgets before they are registered.
• Ensure that non-placeholders (inline arrays instead of string references) can be supplied in starter content.
• Re-use auto-draft posts as starter content across theme switches.
• Introduce starter_content param for WP_Customize_Manager::save_changeset_post() which is false except when starter content is being loaded on a fresh_site.

See #38114.
Fixes #38541.