Join a sandbox Product through a join link

Redeems a join link: creates a sandbox test customer from the first name, activates its enrollment in the link's Product, opens its customer_balance account, funds it with the link's test money, and signs it in. The session is bound to that one customer and Product, and ends no later than the link does. An unknown token is not found, an expired link is refused as expired, and a link that has admitted maxJoins people is refused as full. Each call makes a new customer; the same person joining twice is two customers. The session token is a one-time secret surfaced only in this response.