Product Preview: WebView Authentication (self-hosted sites)

[ealeksandrov]

We can use a nonce to authenticate the preview request for self-hosted sites. (You can see how WPiOS handles the preview nonce in its PreviewNonceHandler.)

• When we request the user's sites from the /rest/v1.1/me/sites endpoint, we can add frame_nonce to the list of options in the request.
• We can save the nonce (a String) in Core Data to use when we need it.
• When we prepare the preview URL to display in the web view, if the site is self-hosted (not a WordPress.com store) we can add ?frame-nonce={FRAME_NONCE} to the URL.