Posts, Post Types: Move `get_post_states()` back to the admin for now, require...

Posts, Post Types: Move `get_post_states()` back to the admin for now, require the file in `WP_Customize_Nav_Menus::customize_register()` instead.

This provides a minor performance improvement by only running the function in contexts where it's needed.

Follow-up to [47211], [47213], [47763], [48619].

See #46829, #49374.
Built from https://develop.svn.wordpress.org/trunk@48620


git-svn-id: https://core.svn.wordpress.org/trunk@48382 1a063a9b-81f0-0310-95a4-ce76da25c4cd