Add maildev

[jenniferplusplus]

Is your feature request related to a problem? Please describe.
Multi-user apps very often need to send email to verify new sign ups, and when performing other account management tasks, or for other reasons. It would be very helpful to be able to test those features in this sandbox.

Describe the solution you'd like
Maildev is a self-contained SMTP server that will capture and display emails sent through it, rather than actually delivering them. It's useful in development, where email is often hard to test and debug, but shouldn't be sent to live inboxes.

https://github.com/maildev/maildev