Persist Stripe ID on sign up

Now that we added a column for the Stripe in in https://github.com/codecation/trailmix/pull/111, let's start using it for new customers.
This string just appearing here seems like the Mystery Guest testing smell.

We could fix it by called `stub_stripe_customer_create` with a parameter of the stripe id.

Cure might be worse than the disease though. WDYT?