Timed question sets

Question set timers should be implemented to allow specific sets to have a time limit associated with them. On attempting the set, current time plus timer should be added to the user's set record. This time marks the latest point at which the question set may be submitted (submission after this point should result in error / locked set until timer is reset).

A separate cron job can be used to look for expired sets and automatically mark all unanswered questions incorrect (nb: this needs some thought as it may result in questions getting inappropriately high SQs some way of flagging the question as unanswered instead - different value in the table, even specifically associated with the set will be required).

Will be useful for mimicking real exams and to allow tutors to test students under 'exam conditions'.