Supporter page: Support a “Expand All” link for collapsible sections

I wrap the entire section that has material that can be expanded in a
div with class `expandable-section`. Once doing so, if you provide an
anchor with the class of `expander`, that anchor will be created with
text in the `data-expand-link-text` attribute.

I've also added some CSS to make the link look a certain way, for
good measure.

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.