New "Preference": lock past due assessments Merged

A common customer request is that some instructors do not want students to be able to submit after the due date has been reached. Others do, though, so this needs to be an option.

We will add another check box in the Preferences Screen:

TEXT: Lock Overdue Assessments
TIP: "If ticked, students will not be able to submit a response after the due date is reached"