Linking to datastream fcr:metadata in HTML UI

Fixes https://www.pivotaltracker.com/story/show/80952408
It would weird me out a good bit if we were publishing, at the same URI, two different RDF representations of the same resource. (And if the HTML is at the same URI modulo conneg, and is providing RDFa, it is an RDF representation of the resource.) Not saying we can't, just saying we should be very, very sure.