General improvements

When you want to modify your registration and want to change your time slot(s), you can basically select multiple time slots.
This is risky, because suppose I have a registration of which I want to change a time slot from X to Y, I have to set the number of participants for time slot X to 0 myself, as it automatically remains on the original entry number, while I also have to choose a new time slot Y (but I can even select multiple time slots).

So the limits for the time slots are not linked to how many people are participating.

This way, mistakes and double bookings will happen, and free places for time slots will no longer be correct.