[WIP] Replace es_app_to_dict with a Serializer (bug 945820)

https://bugzilla.mozilla.org/show_bug.cgi?id=945820

This replaces `es_app_to_dict` with a `Serializer` that inherits from `AppSerializer`, to share as much code as possible with it. This also allows us to provide a simplified version that returns less fields easily. This will make https://bugzilla.mozilla.org/show_bug.cgi?id=958608 trivial.

WIP because there are a few FIXMEs I want to tackle before merging, and I have a couple test failures with content ratings that I need to investigate.