nmbug-status: Style headers with smaller fonts

nmbug-status: Style headers with smaller fonts

We only use h1 through h3, and David prefers smaller headers [1], so
shift over to the font sizes usually used for h2 through h4 [2,3,4].
I haven't bothered with the W3C's default margins, since a bit of
extra whitespace doesn't seem like a big deal.

[1]: id:87k2nl8r0k.fsf@zancas.localnet
     http://article.gmane.org/gmane.mail.notmuch.general/21595
[2]: http://www.w3.org/TR/html-markup/h2.html
[3]: http://www.w3.org/TR/html-markup/h3.html
[4]: http://www.w3.org/TR/html-markup/h4.html