Delete a one-off time slot

Delete an empty one-off slot, immediately. The slot must belong to the signup in the path (a slot of another signup is 404 not_found, and nothing is written). A date of a repeating schedule (its seriesId is set) is refused with 409 conflict: cancel it instead with POST /signups/{id}/slots/{slotId}/cancel. A slot with registrations is also 409 conflict — on a past date too — and nothing is deleted; remove the registrations first. So is a slot with an item scoped to it that has claims: deleting the slot would move those claims to the whole signup without telling anyone (and unlock them on a past date), so delete those items first (DELETE /signups/{id}/items/{itemId}?release_claims=true) or re-scope them (PATCH /signups/{id}/items/{itemId}). Items scoped to the slot that nobody has claimed are kept and become whole-signup items (scope: null) when the slot goes. Deleting by id is idempotent in effect: a second DELETE of the same slot is 404 not_found. Notifies nobody.