[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.
Hmmmm. How do we feel about URL prefixing by repo name vs. passing a serializer property? It feels like the latter scales better, and so long as we maintain backwards-compatibility in the default endpoints (i.e. if no `serializer` querystring param) and in any unprefixed `v1` endpoints, we should still honor the spirit of the previous setup.