Linking to datastream fcr:metadata in HTML UI

Fixes https://www.pivotaltracker.com/story/show/80952408
I have no opinion about the JS stuff, but I don't think the argument on Accept works here. The resource is the resource, and our RDF presentation of it should not vary by MIME type. If we could guarantee that the version with more info (I guess the HTML) RDF-entails the others, I would be less weirded out, but still...