Linking to datastream fcr:metadata in HTML UI

Fixes https://www.pivotaltracker.com/story/show/80952408
The point of pulling in the child node triples is so we can link to child2/fcr:metadata for non-RDF children -- otherwise the description is hidden and the user gets a download dialog when they expected to get the description.

There are probably other ways to smuggle the child type info into the Velocity context, so that's another option to explore.