added accessibility enhancements for details element

Making the details element more accessible by adding role="button" attribute, aria-expanded and screenreader help text. This informs the user that the interaction is a button and it's reveal/hide states. 
Yeah, looks like it's defaulting them closed, which isn't very good IMHO. Might ask Gemma. Still - the polyfill only applies for browsers that don't support details - the rest will get the behaviour as spec'd.