Limit a promo code to one use per member

Let admins set a promo code so each person can only redeem it once, including if they cancel and re-subscribe later.

At the moment, the Usage Limit on a promo code counts uses across all customers, not per person. A member who used a code, cancelled and came back can enter the same code again and get the discount a second time.

Use case: running a quarterly session and giving attendees a discount code that works for 7 days after it. Someone who attends one session, uses the code and cancels shouldn't be able to use a code again when they attend the next session.