Integrate adding of nav menu items with creation of posts

[westonruter]

When adding a nav menu item to a nav menu, if the desired post/page does not exist yet, there should be a way to create it right then and there. With a stub post initially created, there can be a link to flesh out the post in the Customizer. This is the opposite of #51

Depends on #48.
Requires #156.

See https://core.trac.wordpress.org/ticket/34923

[westonruter]

Committed to core (minus the link part, which can be addressed in #147): https://core.trac.wordpress.org/changeset/38436