Validate coupon codes

[justin (Justin DiRose)]

@Kchalk

By allowing the transaction to go through, the user expects a discounted price but will be surprised at the full charge. The occurrence of typos is real, I have seen users fumbling with even the simplest copy-paste instructions.

Coupon code validation is implemented in this PR:
https://github.com/discourse/discourse-subscriptions/pull/74

EDIT: Merged.