If an equipment item is overdue, then it should not be available to check out

Currently, if an equipment item is overdue, then this is its availability:
- Unavailable for the length of the reservation
- Unavailable on current day
- Available tomorrow and beyond

This doesn't make sense. We shouldn't allow users to reserve equipment that the loan office doesn't have, since this relies on hoping that users with overdue equipment will return them as soon as possible. We should make it so that if an equipment item is overdue, then it is unavailable for check out until checked back in.