Implement editing of slots in ClientSchedule

Implement tracking of time slots along with editing and restoration of
singularly edited time slots.  This provides a simpler interface to work
with when rendering tables of time slots that can be edited than
directly manipulating events and shifts containing an array of slots.

utils/functions.ts

@@ -1,4 +1,5 @@
 import type { LocationQueryValue } from 'vue-router';
+import type { Id } from '~/shared/types/common';
 
 export function queryToString(item?: null | LocationQueryValue | LocationQueryValue[]) {
 	if (item === null)
@@ -15,3 +16,7 @@ export function queryToNumber(item?: null | LocationQueryValue | LocationQueryVa
 		return queryToNumber(item[0])
 	return Number.parseInt(item, 10);
 }
+
+export function idMap<T extends { id: Id }>(entities: T[]) {
+	return new Map(entities.map(entity => [entity.id, entity]));
+}