How to clean up job folders in user cache?

[tcompa]

Branching from #474

#1521concerns the server-side folders. A different question concerns what to do with user-side cache folders. Zipping it is not really relevant, as it's meant to be ephemeral anyway (plus it's not complete, since it only has the user-produced files). We could remove it, but it's not great to send some sudo -u user rm commands. Probably the tradeoff for the moment would be to regularly remind users that they need to clean it up. Note that this is mostly relevant if