iarc changed their content descriptors (bug 948156), split descriptors by body in dehydrate (bug 931947)

**Notes:**

- Patch isn't too bad, just a lot of migration lines and simplifying of ```mkt.ratingsbodies.ratingdescriptors``` data structure, some alphabetizing
- IARC changed their Generic content rating descriptors to match PEGI's instead of ESRB's.
- Improve how content descriptors are dehydrated in app resource/es_app_to_dict by keying by ratings body.
- New PEGI icons (TODO: new generic/CLASSIND icons)
- TODO: flue

Blocker for https://github.com/mozilla/fireplace/pull/332