iarc interactive elements backend (bug 936679)

Notes:

- uses ```DynamicBoolFieldsMixin``` which is also used by ```AppFeatures```, ```RatingDescriptors```
- refactor model tests that use the above mixin into one generic test mixin
- set iarc interactive elements from the iarc response (on form save)
- make creation of ```ContentRatings``` opt-in rather than opt-out on ```app_factory()``` (currently waffled)