Add "Apply Coupon Code" to Payment Dialog

[kmcurry]

A coupon code allows us to give free and discounted access to production 3Scape (https://go.3scape.me).

[kmcurry]

Needs Stripe code.

[kmcurry]

Coupon is ready in Stripe. Put in config.js for payment section: config.payment.coupon-vste