Show app release notes if they are available (bug 945852)

https://bugzilla.mozilla.org/show_bug.cgi?id=945852

See also https://bugzilla.mozilla.org/show_bug.cgi?id=945858 for the zamboni side of things, where we let developers add release notes for hosted apps as well.

**Screenshots:**
Mobile view, no updates to show
![screen shot 2013-12-04 at 18 57 16](https://f.cloud.github.com/assets/187006/1675663/c213716e-5d0d-11e3-9fb0-6ee6bd22f361.png)

Mobile view, both description and updates truncated
![screen shot 2013-12-04 at 18 54 22](https://f.cloud.github.com/assets/187006/1675662/c212fb30-5d0d-11e3-9ba2-cf561294b411.png)

Mobile view, description truncated, updates fully shown
![screen shot 2013-12-04 at 18 54 33](https://f.cloud.github.com/assets/187006/1675659/c20ee9a0-5d0d-11e3-8aa0-0e08cadc8eb8.png)

Desktop view, both description and updates shown
![screen shot 2013-12-04 at 18 55 27](https://f.cloud.github.com/assets/187006/1675660/c210e232-5d0d-11e3-8bf9-b0734eeb85c9.png)

Desktop view, no updates to show
![screen shot 2013-12-04 at 18 57 06](https://f.cloud.github.com/assets/187006/1675661/c211dc96-5d0d-11e3-8fc0-b39cb1359f7c.png)