Add payment success page

[Daniel15]

This is a port of Daniel15#3 from v0.22.0 to the latest develop branch. Unfortunately I can't fully test it since I'm hitting errors when trying to create a new invoice.

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Adds a new payment success page that Stripe can redirect to after a successful payment.

• Split PaymentPortal into several files: PaymentPortalPage and PaymentPortalInvoiceHeader with the reusable parts, and PaymentInvoicePage for stuff that's specific to the current invoice page
• Add new route for Stripe success_url: https://example.com/payment/{paymentLinkID}/success/{stripeCheckoutSessionID}. Currently the Stripe checkout session ID is not used, but it could be used to load details about the payment.
• Remove hard-coded localhost success_url and cancel_urls and replace them with proper URLs: success_url goes to the new page, and cancel_url just goes back to the invoice.

Closes #849