feed es view serializers (bug 1036216)

Breaking https://github.com/mozilla/zamboni/pull/2298/ up.

- Extract stuff from ESAppSerializer to a BaseESSerializer that reuses the ```mock_object``` stuff.
- FeedItemESSerializer -> FeedApp/Brand/Collection/ShelfESSerializer -> ESAppSerializer
- The serializers expect ```app_map``` and ```feed_element_map``` in the context. This is because we do batch ES queries in the view and we map back the IDs to ES objects in the serializers.
Can you provide a bit more information about what this offers, how it works, etc in a docstring?