[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.
I prefer the latter, but we were already using the former so I re-used the same idea. I could switch to a querystring parameter and store the list of allowed variants somewhere in mkt/api/.

I didn't like the name I came up with for the parameter, `api_app`. The problem I have with `serializer` is that, in this case, it's actually `app_serializer`, the main serializer isn't changed. Not sure if that's really an issue though.