Cancel a contact's spot

Releases the contact's spot. Inside the location's cancellation window it is a late cancel (`late: true`) that still consumes the covering credit unless `excuse` is true. A freed spot is offered to the waitlist automatically.

curl -X POST "https://example.com/classes/497f6eca-6276-4993-bfeb-53cbbbba6f08/cancel" \  -H "Content-Type: application/json" \  -d '{    "contact_id": "f245e2a3-e141-4d41-912c-b1c1f5bd5598"  }'

{  "attendee_id": "83226cf6-9cc1-4adb-b536-110595637d19",  "class_id": "54be4e86-fa06-45fc-86e5-29ea93b4a062",  "contact_id": "f245e2a3-e141-4d41-912c-b1c1f5bd5598",  "status": "cancelled",  "late": true}