Feature: Tax Regimes are added so that different bills with different series can be printed for Different regimes such as VAT and GST

Chore: Model relationships updated to make them simpler
Chore: Bill printing majorly refactored for it

Due to the sheer depth of the changes. There can be showstoppers. Please test it carefully

bookie/src/app/settings/settings.service.ts

@@ -26,4 +26,20 @@ export class SettingsService {
       catchError(this.log.handleError(serviceName, 'setPrefillCustomerDiscount')),
     ) as Observable<boolean>;
   }
+
+  runMaintenance(
+    startDate: string,
+    finishDate: string,
+    beerFile: File,
+    saleFile: File,
+  ): Observable<boolean> {
+    const options = { params: new HttpParams().set('s', startDate).set('f', finishDate) };
+    const url = '/api/maintenance';
+    const fd = new FormData();
+    fd.append('beer_file', beerFile);
+    fd.append('sale_file', saleFile);
+    return this.http
+      .post<boolean>(url, fd, options)
+      .pipe(catchError(this.log.handleError(serviceName, 'runMaintenance'))) as Observable<boolean>;
+  }
 }