Improve the UX of the Nav Menus screen. Kill the tabs, and change to a
dropdown, unless you have zero or one menus (which is the most common),
in which case you jump right into editing your sole menu.

Do assignment to location using checkboxes in the main menu editing
section instead of the backwards menu => location assignment in a
random meta box.

More to come, but this gets us started.

props lessbloat, DrewAPicture, jkudish. see #23119

git-svn-id: https://core.svn.wordpress.org/trunk@23441 1a063a9b-81f0-0310-95a4-ce76da25c4cd