Warn site admins if they are not accessing Sourcegraph over HTTPS

Created by: nicksnyder

Customer should configure their Sourcegraph instance to be accessible over HTTPS by following these instructions: https://docs.sourcegraph.com/admin/nginx

If we detect that a site admin is accessing Sourcegraph over HTTP (this should be easy to detect in JavaScript, right?), we should show the site admin a warning (I don't have a proposed UI/UX) and point them to the above documentation.