Approve purchasing for an account

Clear an account refused by the new-account purchase gate, so it can complete the purchase it was refused, and lift the freeze that came with the refusal. This is the only way out of a denial - nothing expires automatically - and it also records that a human has looked, so the gate will not refuse the account again. The Slack alert carries a button that does the same thing; this endpoint is the fallback where Slack interactivity is not configured. Approving an account that was never refused is safe: it clears the account in advance.