Coupon Save on Submit Invoice

[elliegoldstein8]

• [x] Calculate discounted amount from each coupon
• [x] Save coupon details (Coupon Name/Code, Quantity, Discounted Amount) to 'coupon_list', a child table of Sales Invoice.

[elliegoldstein8]

Connected to issue #209 for not saving discount amount to Accounting.

[elliegoldstein8]

• [x] Apply coupon discount to grand total
• [ ] Add coupon discount to total invoice discount
• [ ] Apply coupon discount per item