Action: Purchase confirm

[mtnygard]

Write the action for :action.type/purchase-confirm, in namespace simtest.actions.purchase-confirm.

Must POST to /checkout-confirm. In this toy version of the target system, we are not doing a real checkout process. Instead we simulate checkout with a pair of URLs. A POST to /checkout-confirm indicates that we are completing checkout.

Must record the response time, response code and response body.