Refactor ClientSchedule to mutable types

Use a single mutable location, event, slot, etc, for each unique
resource that keeps track of the local editable client copy and
the server copy of the data contained in it.

This makes it much simpler to update these data structures as I can take
advantage of the v-model bindings in Vue.js and work with the system
instead of against it.

pages/edit.vue

@@ -78,7 +78,7 @@
 				<TableScheduleShiftSlots :edit="true" :roleId="roleFilter" :shiftSlotFilter />
 			</template>
 		</Tabs>
-		<p v-if="schedule.modified">
+		<p v-if="schedule.isModified()">
 			Changes are not saved yet.
 			<button
 				type="button"