Improve website security

Score: B

https://observatory.mozilla.org/analyze/www.modcluster.io

PS: If you're hosted on Github Pages, we were able to reclaim 25 points by following this procedure: http://qszhuan.github.io/technology/2015/08/12/add_csp_to_github_blog, and a further 40 points by using HTTPS instead of self-referential links.