Add a download link to the thumbnail.

This one was way harder than it seemed like it should be, and the solution is hacky in some ways, but maybe points to some better solutions for isolating "core-style" changes in our theme. There is actually already a service that grabs a "best" download link and puts it in a meta link in the header, but it is so far isolated from the item page rendering that it seems unreachable (and is in any case handled as a side effect—the link is never stored so it could be queried later). So the bulk of the code here is mostly copied from that. Not the greatest code smell, but it does avoid another core mod.

What I did was create a new component "thumbnail-link", which is loaded in our theme and then called via our item page component. It will be worth looking back at other core mods and seeing if they can be handled in a similar way. That way, the only real risk in applying upstream updates would be name collisions from new components. We could decide to add a "-duke-" or "-dul-" prefix to component names to avoid that.

The download link grabs the best available download candidate and wraps the thumbnail image in a link if one is available. If the item is (e.g.) embargoed, the link is not added.