success message for content ratings (bug 948656)

Contains funky logic. Since content ratings are created through the IARC tool, asynchronous of Django's request/response cycle, we don't have a chance to add a Django message. In this patch, ```request.session``` is used to keep state and determine whether an app had just gotten a rating, and displays the appropriate message. Confusing, but the tests show each case.

- if an app just got a rating and it is now complete, display something like "Congratz, your app is now pending."
- if an app just got a rating, display something like "Your app needs payments" + "Ratings saved."
- if updating an app's ratings, just display "Ratings saved."

![screen shot 2013-12-11 at 8 53 22 pm](https://f.cloud.github.com/assets/674727/1730641/60a1360a-62e9-11e3-88a7-3cc186a1cdd7.png)

![screen shot 2013-12-11 at 8 58 00 pm](https://f.cloud.github.com/assets/674727/1730659/079df1c8-62ea-11e3-85da-4812a0f535c6.png)