Define height for static page modals

![screen shot 2014-10-09 at 5 46 28 pm](https://cloud.githubusercontent.com/assets/667753/4583878/e133bd02-4ff5-11e4-8206-f71819494474.png)

The window scroll still works, but can't (with my research) be disabled easily (without using JS).

Closes #713.