plagiarism_turnitin: Check if rubric has been set before applying it

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>
plagiarism_turnitin: Push back due date on late sync by one day only

Students can't view their GradeMark feedback until after the post date,
which gets set to the due date, which in turn is pushed back (by a
month, before this change) if a student submits after it has passed.
This would result in students being unable to view their feedback until
a month after the last submission. Ideally it would only be pushed back
by an hour, but this might not be enough to allow for long queues to
process submissions on the Turnitin side, so a day should be OK.

Signed-off-by: Tony Butler <tonyjbutler@gmail.com>