Allow listing a project as a derivative of a resource published elsewhere

[bemoody]

It should be possible to list a project as being "derived from" a resource that was published on another site, by providing the DOI (or other URL?)

The parent resources should be shown in the "Parent Projects" info box (though "Projects" is maybe the wrong word to use), and should be listed in the DOI metadata (pull #2119) when the PhysioNet project is published.