[#665] Collapse plugin tweaks

* make the generated button the outermost element, so that we don't have
  to override as much of the font styling

* remove some now-redundant CSS

* tweak the logic of isExpanded so it defaults to start with expanded
  (if no data was given)
[#665] Document the collapse plugin in /dev/style-guide

* which files to include

* how to load (adds a new "onload" block in template toolkit for
  convenience)

* what is important to include: data-collapse, optionally
  data-collapse-state