This requires some explanation.  It turns out that:
a) `formatted_due_date` is the problem due date, not the submission due date.
b) The submission due date needs to be calculated if it's not explicitly set.  We do this, but `self.is_closed()` doesn't return it unless the problem is closed, which means...
c) This message will never be displayed, because if the problem is closed, another template is used instead.

I'm concerned about changing the interface for `self.is_closed()` this late in the game, since it's used in other places as well.  I'm working on those changes now (because it's really important to tell students when the submission deadline is), but I wanted to get this up for review first.