Signup, Management, & Webhook Lambdas for Stripe

[john-osullivan]

Making this PR so it's easier to review work in progress. Overall integration still remaining:

• [ ] Two new API Gateway resources/integrations/responses
• [ ] Verifying permissions on all Lambdas (should they have separate roles?)
• [ ] Writing up IP-based IAM control for webhook Lambda
• [ ] Deploying & testing new payment-gateway-lambda source