Uniqueness of coupon code

[iambibhas]

Right now DiscountCoupon.code are unique only inside the same DiscountPolicy. But here in this bit we assume that coupon codes are unique globally. Which caused couple of failures when we created couple of coupon codes with same name as some old coupon codes.

So there is a confusion about the uniqueness of coupon codes. Are they unique -

1. within a discount policy
2. across multiple discount policies but within a single item (as Item links to multiple discount policies)
3. globally, across all discount policies

We'll need to update our logic accordingly.

/cc @jace @vidya-ram

[jace]

Code needs to be unique to an ItemCollection. This requires two fixes:

1. Enforce that unique constraint when creating codes (may require a PostgreSQL functional index).
2. Join to an ItemCollection when looking up a code.