Nicer "tasks due soon" emails

CORE-655

Original bug turned out unreproducible at this time, but took the opportunity to ensure consistency between the subject line and email body in terms of "tasks due in X days". Also added the "in X days" text to each task in digest email.
Not sure, but fixed to avoid relying on leftovers.

Before it was essentially taking the last item's assignee, now it will take the first.