Add a quiz lockout mode

[dlglin]

This was discussed many years ago for WW2 (and could still be possible there).

Add the ability to flag an exam as being exclusive, and once a student starts an exclusive exam they are not able to view any other sets until they have completed the exam.

We'd have to be careful about what constitutes "completing" the exam. Obvious conditions are:

• The time limit for the exam has elapsed
• The student has used all of their attempts
• The student has received 100% on the exam

There might be more exotic cases that need to be taken into account. For example: an instructor sets up an exam with unlimited attempts; a student starts the exam, but after some number of attempts concludes that they aren't going to be able to get 100% on the exam and wants to go back to working on their homework. In such cases does it make sense to give the student the option to close out the exam, i.e. give them a button that says something like "I understand that by clicking this button I will no longer be able to submit additional attempts for credit".