Recognize `GITLAB_CDN_HOST` environment variable in development

In the frontend, it is sometimes useful to be able to test the CDN settings in order to debug same-origin policy issues like gitlab-ce#60217 or gitlab-ce#50451.

Normally this would be as simple as setting up a dummy hostname within /etc/hosts and then running GitLab with GITLAB_CDN_HOST=http://fake-cdn.com, but that environment variable is only honored in config/environments/production.rb. We should make this work in dev-mode as well.