Book mode: Assume target=_blank when frame-ancestors keeps the content from loading

I've created [a demo book](https://hackmd.io/@patcon/H1p_i2jkS) demonstrating the issue with links to websites like GitHub. GitHub uses the CSP `frame-ancestors 'none'` ([docs](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-ancestors)), which prevents the browser from showing it. This results in a blank page and this error in console:

> Refused to display 'https://github.com/hyphacoop/organizing/blob/master/2019-06-05-all-hands-meeting.md' in a frame because an ancestor violates the following Content Security Policy directive: "frame-ancestors 'none'".

Feeling like maybe it's possible to detect this and open in a new tab. I understand that it might NOT be possible to show the proper "new tab" icon with this method, but perhaps an open source repo with a whitelist of domains could allows users to keep a manual list up-to-date, which HackMD could use to know the CSP in advance. Also, the whitelist could just be a list of test urls/domains, and a script could run nightly to determine the policy on its pages.

Anyhow, thanks for the tool, and any consideration! 🙌

Inizia dal libro demo collegato e riproduci la pagina incorporata vuota usando l’URL GitHub e la relativa policy frame-ancestors. L’issue non indica file sorgente né test, quindi individua innanzitutto i punti di ingresso per il rendering dei link e l’incorporamento in modalità libro. Il lavoro sarà completato quando sarà stato concordato un comportamento per le pagine bloccate incorporate in un frame e sarà stato verificato che i link interessati non lascino più una pagina vuota.

Scritto dal modello di indicizzazione a partire dal testo della issue.