Switch to more explicit CSS styles for boxes in Review Board.

Boxes in Review Board now follow the new convention in Djblets of
prefixing all related CSS classes with "box-" and wrapping the title
element in a "box-head" div.

This matches what we've been using in RBCommons, helping us to keep
things consistent, and prevents conflicts with existing "title" and
"main" CSS classes.

Reviewed at https://reviews.reviewboard.org/r/5754/