Chore: In overlord / sale / bill.service now the BillViewItem is just a view item

The data is kept as the original bill object and this view generated on every change. It has no sanctity.

To deal with the challenges of Selection of items in the bill.component.html created a bill selection item.
This is converted to string while checking else the selection model fails.

Feature: It now checks if Happy Hour items have equivalent regular items in each kot.
Feature: Discount won't apply to happy hour items.
Checks for both are both in front end and back end.

bookie/src/app/sales/bills/bills.component.html

@@ -62,8 +62,8 @@
             *ngIf="!row.isKot"
             [disabled]="!row.id"
             (click)="$event.stopPropagation()"
-            (change)="$event ? bs.selection.toggle(row) : null"
-            [checked]="bs.selection.isSelected(row)"
+            (change)="$event ? toggle(row) : null"
+            [checked]="isSelected(row)"
           >
           </mat-checkbox>
         </mat-cell>