Delete orphaned downloads in data management

[SyntaxBlitz]

I think it's possible for incomplete downloads to be orphaned. I'm not sure, since I've never seen it happen, but it may be worth adding a button (or an automatic cleanup) to make sure incomplete downloads can be removed. They can't be deleted normally (except by deleting the entire course folder, not even by deleting all downloads), since their filenames include .download at the end.