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.
This would read much better if you stored this line in an intermediate variable before passing to the serializer.