Linking to datastream fcr:metadata in HTML UI

Fixes https://www.pivotaltracker.com/story/show/80952408
We have continued to change the default "Prefer" value over time. The HTML view needs a superset of the current default RDF. Is it any less confusing to have some back-and-forth javascript eventually get to the desired RDF than having the HTML conneg provide another default "Prefer" value (in the case that the user did not provide it)?