Nav dropdown for new splash pages

Actual nav & dropdown content not final yet, but this is how it could work.

![dropdown](https://cloud.githubusercontent.com/assets/883598/4321680/2de61330-3f42-11e4-85be-6315e9525bcc.gif)

Have re-used a lot of code from `.team-popover`, but I'm not sure if that'll make it into new design so may not need refactoring for time being.