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 docstring is super unclear. You're deserializing to an app from the app id, and L22-23 could use some context.