Expandable summary

- [new] Summaries are expandable
- [removed] toggle (show/hide) summary on mobile: the logic in the CSS was becoming too complicated. We can put it back later with some more JS instead of relying exclusively on CSS classes if necessary.

![summary-mobile-expandable](https://f.cloud.github.com/assets/85783/1131320/74bccbfe-1bb8-11e3-9468-9cca938f294f.gif)

![summary-ipad-expandable](https://f.cloud.github.com/assets/85783/1131323/78091204-1bb8-11e3-9a5c-4fbb5bae97d2.gif)