supporter: Reorganize form post-input notes.

This makes it easier to keep baseline alignment between labels and their
respective inputs. Declaring `margin-left: 51%;` for the post-input notes
is not exactly what we want, but it's much closer and less troublesome than
dealing with the vertical alignment of labels otherwise.

supporter: New form styling.

This makes better use of bigger screens, and degrades better to smaller
ones.

supporter: Move more presentation rules to CSS.

This commit keeps the presentation basically the same, it just moves
presentation rules out of HTML and into CSS. It's not pixel-perfect but
pretty close.