Cart Discount and Discount Code Improvements

Another round of promotion management improvements:

• "Free Gift" Discounts can now be created in the Merchant Center. When adding a Gift Line Item as a Cart Discount, the price of this item will be 0, and the item will automatically be added to the Cart once the Cart Discount matches the Cart.
• You can duplicate Product and Cart Discounts directly without having to manually recreate all settings.
• You can control the "validFrom" and "validTo" settings of both Cart Discounts and Discount Codes now. This helps you to manage and plan promotional campaigns in advance.