Replace deprecated Stripe Sources API with Stripe Checkout

- Add stripe/checkout.py module for Stripe Checkout Sessions
- Add Payment model to track completed payments
- Update billing page with pay-what-you-want, annual, and top-up options
- Keep pay_what_you_can preferences, add Pay Now button for actual payment
- Add webhook handler for Stripe events
- Remove old card management code (deprecated Sources API)
- Update stripe requirement to >=5.0.0

development.ini

@@ -89,10 +89,11 @@ app.theme = meta
 # choices enabled or disabled. defaults to disabled.
 #app.stand_alone_mode = disabled
 
-# stripe: credit card storage and processing.
+# stripe: Stripe Checkout payment processing.
 # This syntax will automatically expand an ENV var of the same name.
 app.stripe.secret = ${REMARKBOX_APP_STRIPE_SECRET}
 app.stripe.public = ${REMARKBOX_APP_STRIPE_PUBLIC}
+app.stripe.webhook_secret = ${REMARKBOX_APP_STRIPE_WEBHOOK_SECRET:-}
 
 # slack: bot notifications.
 app.slack.secret = ${REMARKBOX_APP_SLACK_SECRET}