Use Cloudflare CDN for GitLab.com

Report gitlab-com/operations#42 (comment 3588320)

"Have you looked into using a CDN (I'd strongly recommend CloudFlare, I use it for my Rails apps)? Could be a huge performance improvement and take significant pressure off your servers. I realize it's not open source, but seeing as the CI Builds run on Digital Ocean and Azure, that's probably not a huge problem."

Sid: Would also be great to help counter DDOS attacks, I think CloudFlare is a great choice