Apply promo code and discount value based on radio option

[yoowon90]

• Add promo_code as valid field for a wishitem
• If a valid value for promo_code exists, display radio (or a toggle button) next to wish item (e.g. by toggling show_discounted_price)
• If no promo_code exists, do not display radio
• If show_discounted_price, change price
• If not show_discounted_price, change back price to un-discounted value ( e.g. new_price = old_price * 100 / discount
• Purchase list will display the price that I bought at (may be a future enhacement: confirm price when purchase button clicked