CDN check failure should not throw up a warning

[eloquence]

Per the original spec (#488), use of a CDN like Cloudflare should not cause a warning (moderate or severe), provided no assets are loaded from a third party domain. We still consider CDNs like Cloudflare problematic for SecureDrop landing pages (and the official recommendation in the docs is "consider avoiding"), but there are other security reasons news organizations may prefer to use them, and on balance, we're not making an effort to discourage their use (indeed, we use Cloudflare for SecureDrop.org).