capability to download raw (original upload) image for a particular artwork

[slivingston]

From the perspective of a user, there should be an easy means to download the raw image of an artwork, i.e., the original image file that was uploaded using the artist-portal.

Practically, there are two components:

1. implement a button within the UI, e.g., in the details pop-up per artwork, that leads to an AJAX request;
2. provide a backend route that leads to a file download response (MIME type application/octet-stream) or a URL from which the file can be downloaded using a separate request.

This task is related to #41

[slivingston]

more important than this feature is the ability to download print-files. As such, #36 should be completed together with this issue.