WIP: Refs #14913 fix voucher order count

Fix discrepancy between voucher num_order and order discounts shown in Voucher dashboard.

Few possible reasons why num_order might be out of sync with actual number of orders where it was used.

1. Voucher was used but the basket was not applicable for discount (See: Issue https://thelabnyc.plan.io/issues/14913)
   • This will increase the count in num_order but will not create a OrderDiscount object
2. Deleting order records which had used the Voucher
   • Deleting an order will delete OrderDiscount associated with it due to cascading

Changes

• ignores the value stored in num_order and computes the order count for OrderDiscount model

Alternative Approach

• Manually re-sync num_order property with OrderDiscount using a code snippet similar to one below
  • We already have a fix in place for Problem 1
  • As long as we never encounter Problem 2, this will be a better fix

    voucher.num_order = OrderDiscount.objects.filter(voucher_id__in=ids).count()