Add sandbox Paypal Integration

[dunkOnIT]

This issue tracks the steps required for having a working PayPal Integration in a test environment. Included in the scope of this is a refactor of the existing Stripe/payments logic, so that it becomes much easier to add more payment providers in the future.

Overall Steps

• [x] Add ability for organizers to link paypal accounts
• [x] Refactor Stripe to use the same pattern as paypal account linking
• [ ] Refactor StripeTransaction to StripeRecord
• [ ] Refactor StripePaymentIntent to be a generic column with a polymorphic association to a Stripe/Paypal Record object
• [ ] Process Paypal payment confirmations via a generic "payments engine"
• [ ] Add refund capability to payments engine
• [ ] Add any other feature requirements to payments engine
• [ ] Migrate Stripe to use the generic "payments engine" (make sure we have good test coverage on payment functionality by this point)
• [ ] Various polish/cleanup PR's (see backlog)

Backlog

• [ ] Refactor stripe connection stuff into Stripe interface
• [ ] add a validation preventing the organizer from linking multiple integrations
• [x] Add logic in paypal_interface for appropriately converting amounts based on currency code
• [ ] add API endpoints for initiating paypal payments
• [ ] Get paypal payment form working
• [x] Split Stripe payment form into a separate component
• [ ] change Registration Service to use new /payment/init
• [ ] Refactor where PaypalRecord objects are created, and how they have payloads populated (should they be created in PaypalInterface? It is more appropriate for them to be created in the controller)
• [ ] Address all TODOs and notes

Polish

• [ ] Rename StripeRecord:api_type to StripeRecord:record_type
• [ ] Store the user_id of the user who connected the account on the integration
• [ ] change "Payment Setup" icon to be a generic cash sign, not Stripe
• [ ] Integrate Stripe and Paypal setup in payment_setup page (ie change text and button positioning to be more intuitive)
• [ ] Transition payment_setup to React?
• [x] Create separate database fields for paypal-specific data
• [ ] Add payment webhook as backup for return_url being interrupted
• [ ] Add onboarding webbook as backup for return_url being interrupted
• [ ] Is there a way that we can allow people with non-supported currencies to use paypal? (eg, converting any non-supported currencies to USD when creating the order)
• [ ] Paypal form error handling
  • [ ] Monolith fails to make connection to paypal
  • [ ] Paypal SDK load fails for some reason (probably incorrect currency)
  • [ ] Prevent organizer from adding more than 1 payment integration (ie, can't have Stripe and Paypal connected at the same time)

Paypal payment form polish

• [ ] Don't submit the form on Enter
• [ ] Does it even need to be a form for Paypal? Maybe we should remove the form components and display the data differently
• [ ] Uncheck the "Donation" tickbox on page reload