Pages created as theme starter content aren't saved if a page is created from Customizer

[dlh]

To replicate on a "fresh site" running Twenty Seventeen:

• Open the Customizer, and navigate directly to "Theme Options"
• Use "Add New Page" to add a page to a front page section
• Publish the changes

The pages intended to be starter content will still be auto-drafts. The links to those pages will still be in the menu; returning to the frontend and attempting to visit them results in a 404.

[westonruter]

@dlh excellent catch! The problem is that when starter content is applied, the nav_menus_created_posts setting is initially non-empty. Nevertheless, the nav menus logic is expecting that it would be empty as seen in the insertedAutoDrafts array which would keep track of the newly-created nav menus in the current customizer session. Once saving, it was intended that this would be used to empty out the list but the logic is faulty. We should only be manipulating the one setting and not trying to keep a separate copy around.

[dlh]

39071.0.diff​ fixes the issue for me.

[westonruter]

In 39502:

Customize: Prevent posts/pages imported via starter content from being dropped when adding post/page stubs via nav menus and the dropdown-pages control.

Props westonruter, dlh for testing.
See #38114, #34923.
Fixes #39071.

[westonruter]

In 39503:

Customize: Prevent posts/pages imported via starter content from being dropped when adding post/page stubs via nav menus and the dropdown-pages control.

Props westonruter, dlh for testing.
Merges [39502] onto 4.7 branch.
Fixes #39071 for 4.7.