Add payment processing UI

Implement the UI views and templates for payment processing, as documented in the django-payments docs.

• Add the view described in the docs above, which shows the payment form or redirect to the payment backend
• Change the invoice detail view to do the following:
  • Display basic invoice and payment information, e.g. in three cards in a row:
    • Details of customer (billing information)
    • Payment method (with a logo/icon if available)
    • Payment status (with an icon and colour designating the status)
      • If the status is PaymentStatus.WAITING. PaymentStatus.REJECTED or PaymentStatus.INPUT, add a button to the do_payment view
• Split the "payer/billing details" and the "payment status" in separate permissions/rules
  • By default, allow anyone who knows the URL to see payment status and payment method
  • Allow only administrators to see the purchased items and the payer/billing information