feed iarc descriptors and interactives through app resource (bug 931947)

** Commit 1**

- Expose IARC content descriptors and interactive elements in ```app_to_dict``` and ```es_app_to_dict```.
- When indexing ratings/descriptors/interactives, index them by the name defined in constants. On the way out of ES, dehydrate by looking up the class and evaluating the lazy strings.
- rename ```slug``` to ```label``` for iarc constants
- Accompanied by Fireplace and Flue PRs

**Commit 2**

- Attach age field to rating constants
- Dehydrator helper function in ratingsbodies constants to build strings and force translation of strings
- Rename some rating names (e.g. "3+" to "For ages 3+")

Fireplace  https://github.com/mozilla/fireplace/issues/304
Flue https://github.com/mozilla/flue/issues/7