[node] Add email to the backend

[bruderstein]

Add an email library to the backend. You can assume there is a reliable SMTP server running locally to deal with retries etc, so you can just fire and forget.

It would be useful to have a sendEmail function wrapped up for other parts of the application to use. The configuration should come from the existing config setup.

It would be nice anywhere in the application to be able to do:

sendEmail({
  to: 'test@example.com',
  subject: 'Validate your email address',
  text: 'Please validate your email by following this link: ' + validationLink,
  html: htmlVersionOfTheEmail
});

This function should ideally return a promise that resolves when the email has been sent, or rejects if it fails.

The from should come from config. As the SMTP server will be local, there's no need to have SSL etc, just a plain ordinary SMTP server connection.