AA-261: Overridding Show Answer functionality for PLS courses

We noticed that since users can choose to reset their due dates,
they would have the ability to let due dates pass and then for any
assessment that allows viewing the answer after the due date would be
visible. The user could thus view all answers and then reset their
due dates to receive a perfect score.

This PR works to fix that issue by changing all show answer values
to not take into account being past the due date when inside a PLS
course.