Students should not be able to book multiple timeslots for the same assignment
Apparently timeslots are like toilet paper: some people start hoarding them, leading to a shortage if the amount of timeslots was calculated close to the amount of students in a course.
Would also resolve partially the need for #293
We don't want to enforce one slot per student per course, as this would block students from claiming a timeslot for assignment 1 and one for 2. Remaining question is then: is there any course setup where we would want to allow students to book multiple slots for the same assignment?