Fix admin order coupon url

On Admin Order screen in the order totals meta box, when a coupon is output where the ID is not found, the coupon name is wrapped in esc_url() which inserts an unnecessary "http://" before the coupon code. I went ahead and converted this line to use add_query_arg() instead and moved esc_url() to the href where the line of HTML is being output.

Oops. It looks like I tried to merge this with Master. Sorry, I wasn't sure which branch is the latest, other than the release-2-0-20, but I didn't know how to choose that. Sorry again, let me know if I need to resubmit this.