Customize: Prevent posts/pages imported via starter content from being dropped...

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.

Built from https://develop.svn.wordpress.org/trunk@39502


git-svn-id: https://core.svn.wordpress.org/trunk@39442 1a063a9b-81f0-0310-95a4-ce76da25c4cd