Add a relation between Coupon and OrderdItem to accurately track coupon usage count

[umgelurgel]

Right now an OrderItem is created with the coupon's code - which means that if the code is reused the following year (e.g. for returning volunteers), the usage count includes past years' use of the code.

Needs to be updated in Coupon.valid and CouponAdmin.usage_count

[artcz]

👍 I think there's even TODO for it in the code.

[umgelurgel]

This will require some insight into how coupon codes are currently generated, as it looks like they might get dropped or updated every year, which we probably want to avoid.

[artcz]

There are management commands to generate them, except for volunteers coupons because those are made either manually in admin or in the shell. Similar story for finaid. Afaik they are not dropped.