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.