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
just an idea... but since the mappings don't differentiate between a list of the given type or a single item of that type, one way we can provide a hint is to pluralize fields that we plan to put lists of stuff in. If we followed that rule, this would be `search_names`.