Meta: Update the dfn panels to match Bikeshed's output more closely This change removes the HTML-specific styles dealing with the dfn panels, and changes html-dfn.js to have those panels match Bikeshed's output more closely. The particular changes are: - Panels have the "dfn-panel" class, rather than "dfnPanel". They also have the "on" class, to make them visible. - Moved panels gain the "activated" class, instead of gaining inline styles. - The elements that can open a dfn panel when clicked now include headings with the "data-dfn-type" attribute (corresponding to a "dfn" attribute in the source). See whatwg/whatwg.org#343, which adds Bikeshed's dfn panel styles to standard.css.