bug:file download

[CommanderStorm]

While the concrete bug causing our backend to shut down is not quite apparent, this might shed a bit more information.

Concrete changes:

• updating the downloads field
• wrapping the whole thing in a transaction
• Separating functions into testable units and adding tests
• not downloading large files to memory and then dumping them to file (instead download them to file and then read them) ^^
• making sure that already existing and not as finished marked files (i.e. partial downloads) are correctly handled