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

- 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.
- TODO: perhaps rename some fields to be less confusing (```slugs```, ```body_slugs```?)