Create a license upon invoice.paid event

[steven-fox]

Added invoice.paid to our production webhook events on Stripe.

I tested this locally for:

• a new user performs a checkout and pays successfully (license created)
• a new user performs a checkout, payment fails, and then payment succeeds (license created)
• a user performs a checkout, payment fails, and then they use the billing portal to pay the invoice at a slightly later time (license created)
• an existing user that doesn't already exist on stripe yet already has a license (old anystack contacts) performs a checkout and pays (new license created)

I can still think of some edge cases this won't handle. I will get going on those as soon as I can.

Also... definitely some refactoring that needs to be done to clean this all up once we've hit a steady state and our logic stops changing. There are a lot of code smells in here from moving fast that I'm not too happy about.