Handle conditional rendering of the payment form

[stephtaylor]

Use case: want to conditionally render credit card inputs, but always display the "submit" button.

ElementNotVisibleError: SqPaymentForm element with id 'sq-payment-form-sq-card-number' is not visible. Does it or a parent element have 'display:none'?

Originally posted by @siakaramalegos in #41 (comment)