Style .box-head separately from .box-title.

The .box-title class was being styled with a border and background, but
this really belongs in .box-head. By doing this, it's easier for us to
change the style of one or the other, and to add additional elements
inside of .box-head.

Reviewed at https://reviews.reviewboard.org/r/5755/
Merge branch 'release-2.0.x'