Add curved box borders to advanced post options

[ketsugi]

Currently in the Write Post page, the fixed-width boxes on the right have curved borders. The flexi-width advanced options boxes at the bottom do not.

[ketsugi]

Submitted patches and images to fix this issue (please ignore formatting.php.diff, I attached the wrong diff file -_-)

Also incorporates #2894 to keep the curve border on the bottom when section is closed.

[ryan]

(In [4001]) Nice curves. Props ketsugi. fixes #2896

[ryan]

(In [4002]) Nice curves. Props ketsugi. fixes #2896