Improving coupon code queries

[kimcoleman]

All Submissions:

• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Updating all coupon code / discount code queries to reverse sort the order so it always gets the latest codes. Also adding filters to each module to allow a larger limit (but defaulting to 5,000 codes).

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

• ENHANCEMENT: Now showing the latest coupons or discount codes in select dropdown to help sites with a large number of codes.
• ENHANCEMENT: Added swsales_pmpro_discount_code_limit filter.
• ENHANCEMENT: Added swsales_edd_discount_code_limit filter.