As a journalist, I want to be able to export all messages and files associated with a source, so that I can handle archival and other post-submission work as I see fit.

"Export" in this case means creating a copy, e.g., on a USB drive.

There are two types of export workflows we should support:

• exporting a single file
• exporting an entire source (all messages/files)

Ideally we should support both in the first iteration, but if we have to compromise, we can start with single file export.