call set_storefront_data before app goes live (bug 931945)

**Notes**

- Uses ```lib.iarc.client.Client.Set_Storefront_data```.
- Call ```Webapp.set_iarc_storefront_data``` on ```process_public_immediately``` on Reviewers side, and ```publicise``` on Developer side.
- "Deserialize" descriptors and interactive elements from our representation back to IARC by reversing the original mapping in ```lib.iarc.utils```.
- Switch on ```iarc``` waffle in Reviewers ```TestReviewApp``` test suite.
- Fix status page to indicate whether an app needs a content rating.
- Define ```iarc_name```s, strings on how the IARC represents Ratingsbodies and Ratings in ratingsbodies constants