Restrict max height of feed preview to the appx height of the Flame's screen, less Firefox OS and Marketplace chrome.

Per Krupa's request. Something something "the fold" something something.

r? @ngokevin

![screen shot 2014-07-24 at 6 14 59 pm](https://cloud.githubusercontent.com/assets/23885/3696081/729b94e0-1388-11e4-8ba6-002eb0efa361.png)