Add "add to home screen" icons for shared items [finishes #75058538]

- Add flappy icon for flappy levels
- Add code.org logo for other shareable levels

![image](https://cloud.githubusercontent.com/assets/206973/3696773/de25a842-1395-11e4-9ecb-12df07377410.png)

does ActionView::Helpers::asset_url not work?