Provide a way of previewing release notes privately

[pmac]

Some releases are more sensitive than others, and @pascalchevrel and others would like a way to preview their new release notes in a way that won't tip off the media. We could potentially:

1. Password protect (basic auth) the /firefox/{version}/releasenotes/ pages on www-dev
2. Provide a different server specifically for release notes preview that can be password protected
3. Provide a better way to locally preview notes
4. Provide a better protected preview some other way
5. Copy the templates to Nucleus and provide a preview there behind the admin auth

Let's discuss how best to accomplish this.

[maureenlholland]

Throwing potential Contentful integration into the ring