customer url param when loading this page to use an existing customer.
Note: Email input is used for billing, which is not the email attached to the Stripe Customer. Please update server.js if you want to create a Stripe customer with your own email address
This demo is currenlty hardcoded to use the instant verification_method. Adding that
as a url param would be a good follow-up; it would require inputs in the microdeposit and
skip cases.