slim down content ratings api responses(bug 982991)

Now looks like ```"content_ratings":{"body":"generic","rating":"3","interactives":["users-interact","shares-info","shares-location"],"descriptors":[]}```

- Infer which rating body to use in Zamboni, not client side, which simplifies things
- Remove description
- Shorten key name
- Pass in slugs only, not translated strings
- Remove some dupe coverage tests