Add HTTPS support

[jamessynge]

We should have some means of authenticating users, and this means we need some means of securing our connections. At a minimum we should have a self-signed cert for debugging, but it would be best to use something like letsencrypt.org to get trusted certs.

See also:

• https://certbot.eff.org/docs/
• https://github.com/certbot/certbot