use ES for feed filtering (refactor)

We got a new fancy filter in WebappIndexer.get_app_filter, let's use that.

- Removes any in-view filtering we did and any fields we added to the serializers for that.
- Allows us to do app limiting in the serializers again.
- Uses a common ES filter that's shared with regular and reviewer search.
- I did made the AppESFieldserializer default to the Fireplace one, since Fireplace is our main use case and largest consumer. Didn't want to have to sprinkle the app_serializer='fireplace' everywhere or risk accidentally having ever Fireplace using the full serializer.



Wait, doesn't it mean that you are doing the job twice if you need both the apps and the count on a serializer ?