Add support for previewing or downloading existing metadata

[thatbudakguy]

There are several references keys that can point to either XML or HTML metadata at a remote location.

In GeoBlacklight, we have a few classes that make the request to fetch the metadata and then parse and reformat it using GeoCombine XSLTs, which all happens in Ruby outside the context of the viewer.

It's an open question whether we want to tackle this in ogm-viewer, especially since we might need to re-implement some of the "transform this into HTML" logic.

One easy option is to just take any metadata that is already in HTML (references key http://www.w3.org/1999/xhtml) and dump it into a new panel in the sidebar. Not sure how many records provide this, but it seems easy enough to do.

For the other XML-based formats (FGDC, ISO19139, MODS), we could at least offer download links to the files, if not actual previews. Sometimes these offer a lot more detail and granularity than the metadata in the OGM record, which is nice to preview if you can.

[thatbudakguy]

Now partially done; we at least have links to view them in the links panel.