Registration feature request: pay when accepted

[timreyn]

Is your feature request related to a problem? Please describe.
We've received complaints from competitors registering for the Worlds waitlist that they need to put down a substantial amount of money to hold a spot which they won't know if they'll get for a few months. I think this is a fair criticism -- we're currently holding a substantial amount of money from unaccepted competitors.

Describe the solution you'd like
Stripe has an API for collecting payment info now, and charging later: https://docs.stripe.com/payments/save-and-reuse?platform=web&ui=elements

The proposed flow would be:

• competitor needs to submit payment details to join the wait list
• we would save a SetupIntent with the registration
• when the person is accepted, attempt to charge their payment
• if it succeeds, accept them
• if it fails, email them to tell them they need to submit a new form of payment, and move on to the next wait listed person