Invoicing enhancements

[allanlasser]

This update resolves two issues with how we handle invoices as a payment method:

• Resolves Support purchasing annual plans with immediate payment #415 by supporting credit card payments for annual plan. It provides a new payment choice in the Plan payment form, "Pay by invoice." When explicitly selected, an invoice will be issued to the customer for payment. It also uses invoices as a fallback method when a card token is missing.
• Resolves Handling for overdue invoices #461 with lifecycle tracking and automated handling for overdue invoices. This brings invoice-based payments for annual plans to parity with credit card payment monitoring when it comes to overdue payment or cancellation. It also adds some useful features for admins to observe and interact with invoices over their lifecycle.

For a detailed list of changes and instructions for manual testing, see our release plan in Notion.

This is a complex PR with an external dependency on Stripe webhooks, so it may take some time to thoroughly review this.

[allanlasser]

@duckduckgrayduck flagged some unclear testing steps when it comes to Stripe webhooks; I'll be cleaning those steps up today.