Refactor `StripePaymentIntent` to generic `PaymentIntent`

[dunkOnIT]

Steps

• [ ] Add a polymorphic association to a :payment_record
• [ ] Refactor stripe-specific method to be generic (use the same logic and just detect what the association is? Similar to what we do for CompetitionPaymentIntegration?)

Questions

• [ ] What is the user_id field for on StripePaymentIntent? The user who is paying? (I suggest a rename to make this explicit, unless its a foreign key I guess?)