Security libraries

[codeofmochi]

Add and configure security libraries that add defenses out of the box ;

• HTTP headers : https://helmetjs.github.io/
• XSS sanitizer ; https://github.com/cure53/DOMPurify
• Rate limiting: https://github.com/animir/node-rate-limiter-flexible

More Express security best practices: https://expressjs.com/en/advanced/best-practice-security.html

[jonscanzi]

Might need to also add some of these protections to the nginx proxy