Access to Memory (AtoM)

The portion of showDownloadComponent that handles constructing a path to be used as the target of the digital object link makes an assumption that all paths are contained on the server on which AtoM is running, and constructs a path via `public_path()`. This breaks URLs for objects imported via URL, since their "path" is actually the URL to their source.

For instance, the generic download thumbnail image would link to "http://www.atom.com/http://www.example.com/foofoo" instead of to "http://example.com/foo".