Turn the Nav Menu meta boxes into an accordion. Less sprawling and overwhelming.

* Registration stays the same — they're meta boxes
* Call `do_accordion_sections()` instead of `do_meta_boxes()` and they render as an accordion

props DrewAPicture, lessbloat, jkudish. fixes #23450. see #23449

git-svn-id: http://core.svn.wordpress.org/trunk@23707 1a063a9b-81f0-0310-95a4-ce76da25c4cd