Hygiene: Toggle classes

Use more meaningful classes given toggling can apply to things other
than sections.

Whilst there use standard hyphenated classes, remove the old school camel
case classes

Use section-heading to apply styles specific to headings (note expanding
the table of contents before this patch made it grow horizontally by a few
pixels)

Note: No HTML changes so therefore no impacting of caching (Alpha doesn't get
cached)

Also:
* Fix categories markup, it hadn't been updated
* Move css rule into client-js given that collapsible-block class is only
added with JavaScript

Change-Id: I309980984d24746c2a7f65bca993e3c306439168