create Shopify one-time swag codes

[tzmanics]

• hit the shopify API
• check that achievement criteria is met
• send code back (possibly to the activity hub (still in questions)) and store it (for issue #456)

[jlengstorf]

we'll need to create a PriceRule that has all the logic for the given achievement

it appears we also have to create a discount code using the DiscountCode API endpoint that follows the price rule, which I want to test because it seems odd to have to do it twice if both are required

• this can happen in a serverless function
• the function MUST validate that the given user ID meets the criteria to create a new discount code
• the function MUST check to see if an existing code already exists