Sticky container titles

A zero-audience AB test. Turn on by adding: `#ab-StickyContainerTitles=show`

![sticky-container-titles](https://cloud.githubusercontent.com/assets/1147913/5541087/534c045c-8acb-11e4-9576-2c814c2bcbb9.gif)

Titles are clickable, causing a jump to that container; thereafter they have their normal click behaviour.