Stash a submission for use on multiple computers (#329)

This allows users to send a new submission with state = 'stashed', which can be later retrieved to continue work on another computer. This is designed to be a simple swap-space for exercises in-progress. 

Stashed submissions will not be assigned to an exercise and should not show up in a list of submissions. When a new file is submitted to a stash, the old one is deleted.