improve SSL security rating for HTTPS

[bhpayne]

https://www.ssllabs.com/ssltest/analyze.html?d=derivationmap.net

as of 20200513 the site gets a "B" rating.

See https://blog.miguelgrinberg.com/post/running-your-flask-application-over-https for guidance.

[bhpayne]

ran

$ date; time openssl dhparam -out dhparam.pem 2048; date

which took less than 1 minutes and added dhparam.pem to nginx

[bhpayne]

Good context: https://www.keycdn.com/blog/perfect-forward-secrecy

[bhpayne]

for guidance, see "2.3 Use Secure Cipher Suites" in https://github.com/ssllabs/research/wiki/SSL-and-TLS-Deployment-Best-Practices

example configs for nginx:

• https://coderwall.com/p/ebl2qa/nginx-ssl-config-with-forward-secrecy-enabled
• https://www.howtoforge.com/ssl-perfect-forward-secrecy-in-nginx-webserver