[WIP] Serialize /feed/<element>/ differently for fireplace/discoplace (bug 1061206)

https://bugzilla.mozilla.org/show_bug.cgi?id=1061206

WIP, this needs tests, I'm submitting this to get feedback on the idea first.
We could certainly add to `MultiSerializerViewSetMixin` (or something similar) functionality that lets you specify the app serializer in the same way:

    def get_app_serializer_class(self):
        app_serializer = self.request.GET.get('app_serializer')
        try:
            return self.app_serializer_classes[app_serializer]
        except KeyError:
            return self.app_serializer_classes['default']