Tweaks to the accordion:

* Enqueues JS in `do_accordion_sections()`.
* Top and bottom rounded corners for the nav menu accordion.
* Better RTL and no-JS.

props lessbloat, DrewAPicture, aaroncampbell, helen. fixes #23449.


git-svn-id: http://core.svn.wordpress.org/trunk@24680 1a063a9b-81f0-0310-95a4-ce76da25c4cd