Allow to reuse coupon number of past promotions

For now we have a constraint to have only one number of coupon per company. But overtime a company creating frequently coupons may reach a point where it become difficult to generate unique number that are still small and meaningful. These properties for coupon are important because often these numbers are printed on ads and fliers and they are entered manually on website or POS.

So I propose to exclude from the unique constraint the inactive promotions.