ES feed view (bug 1036216)

**How it works:**

- ES feed view (three ES queries: one for feed items, one for feed elements, one for apps)
- In ES, feed item index reference feed element IDs. Feed element indices reference app IDs.
- FeedItemESSerializer -> Feed(App/Brand/Collection/Shelf)ESSerializer -> ESAppSerializer

**New useful stuff:**

- Add unindexer helper function to BaseIndexer
- Add a BaseESSerializer (used by ESAppSerializer, FeedXXXESSerializers)
- Attach stuff directly within Webapp extract_document rather than needing an IndexingTransformer beforehand

**TODO:**

- use much simpler app serializer for feed
- have webapps.tasks use BaseIndexer.unindexer
- index app groups