Time slot shows wrong numbers if people retracted their request

Summary

Timeslots show x/y meaning x slots out of y have a request, but x includes retracted requests.

Steps to reproduce

1. A student makes a request for a time slot
2. The same student retracts their request
3. Staff goes to a lab overview page
4. Staff still sees 1/y instead of 0/y for that timeslot.

Expected behaviour

Only show how many slots are currently actively booked.