Make API /calendar endpoint mark slot status correctly

[amcewen]

At present, although it runs a query on the bookings table, the API returns all slots as having the status available. It should check that actual bookings and set the status to available or booked.

AC:

• If there is an entry in the bookings table for a certain time, all slots that overlap with that time should have their status marked as booked (i.e. for laser bookings where there are parallel full-day and half-day slots, a booking in one should also result in the corresponding slot being marked as booked)
• Slots which don't overlap with the times of any bookings should have the status available