Update a time slot

Partial update — supplied fields only. Use null to clear nullable fields. The slot must belong to the signup in the path (a slot of another signup is 404 not_found, and nothing is written). customField is the slot's own question (null removes it, so a date of a repeating schedule asks the schedule's question again). maxParticipants below the number of spots already taken is refused with 400. A date of a repeating schedule (its seriesId is set) cannot have its startTime edited onto another day of the signup's timezone: that is 409 conflict and nothing is written — move it with POST /signups/{id}/slots/{slotId}/move instead. A time edit that keeps the start on the same local day (including any endTime change) is allowed, as is any edit of a slot that is not part of a schedule.