Add cancelled field to event slots

Make it possible to represent one slot out of a multi-slot event being
cancelled by adding a field for it in the slot, in addition to the
existing field on the event itself.

components/TableScheduleEventSlots.vue

@@ -318,6 +318,7 @@ function newEventSlot(options: { start?: DateTime, end?: DateTime } = {}) {
 		start,
 		end,
 		new Set(newEventLocationIds.value),
+		false,
 		new Set(),
 		0,
 	);