Menus: Use `get_post_states()` to denote special pages on the added menu item accordions.

Front Page, Posts Page, or Privacy Policy Page should now be marked as such on the added menu items.

Props garrett-eclipse.
Fixes #49374.
Built from https://develop.svn.wordpress.org/trunk@47211


git-svn-id: https://core.svn.wordpress.org/trunk@47011 1a063a9b-81f0-0310-95a4-ce76da25c4cd