Make content_element credits match trait-credits.json
Make content_element credits match trait-credits.json

The story-fixture-tiny-house.json example validates.  But there is a
credit element inside an image content_element.  It should match the
format of all other credit elements.  But since only id, type, channel
are defined for content_elements it validates.

washingtonpost/ans-schema,jon-peterson/ans-schema