Allowing pulling cover images from Evergreen

[gmcharlt]

It would be convenient if Aspen had an option to take the cover image for ILS-sourced editions and works from Evergreen. In particular, this would allow cover art that is uploaded to Evergreen to be reused by Aspen without needing to upload it into both systems.

One wrinkle: while it's very easy to grab any covers that Evergreen has, there's currently not a direct way to distinguish between images that were uploaded to Evergreen versus covers that are coming from a third-party content provider.