improve header styling
improve header styling

Remove the borders, use better sizes, don't let letter-spacing inherit
past h1, use same margins as paragraphs.

Forthcoming (though I think relatively nondependent) commit coming on
Aloha-Editor

oerpub/github-bookeditor,oerpub/bookish,oerpub/bookish,oerpub/github-bookeditor,oerpub/github-bookeditor