Lambda Utility Functions

Repo for Lambda functions used in the current AWS setup:

• cloudfront-error-204 is a Lambda@Edge function used to convert CloudFront error responses on vector tiles to 204 instead of 404, preventing an issue where bubbles are duplicated between zooms.
• mailchimp-subscribe is used to add emails submitted via HTML form to a MailChimp list and return a 302 redirect to a predefined URL.