Migrate from nodemailer to mailgun-js

Using`nodemailer` is opaque since it's one level removed from `Mailgun`, my email provider. I should look into using more specific packages, e.g. [mailgun-js](https://www.npmjs.com/package/mailgun-js), which would let me reap some more benefits, e.g. scheduling emails, retrieving the list of emails that I've sent, etc.