Send a "thank you for signing up" mail after a user has RSVPd

use either Mailchimp or Turbine's account to send that mail. 
Have a nice mail template. If it's a coding.earth meetup, we can use Turbine's mail server (private crendentials available / needed), otherwise use e.g. Mailchimp credentials. Feel inspired by https://gitlab.com/elmariachi111/devday.io/-/blob/master/src/lambda/submission-created.js