feed item indexer for es feed (bug 1036216)

Breaking https://github.com/mozilla/zamboni/pull/2298 up.

- Index the FeedItem -> FeedApp/Brand/Collection/Shelf -> Apps relation in ES using denormalization "schema"
- FeedItem store FeedApp/Brand/Collection/Shelf IDs
- FeedApp/Brand/Collection/Shelf store app IDs
- Some special logic for grouped feed collections

Related serializers at https://github.com/mozilla/zamboni/pull/2313
should we move `INDEXES` to settings? Or constants?