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.



Also, `group` needs a test.