Allow to bypass sanitizer for external rendering

[sapk]

Before doing a PR that may impact security I would like to have your advice

I would need to bypass the sanitizer by adding an option to external markup like DISABLE_SANITIZER.

The goal is for issue like #5979 to be able to load JS libs (external or in public) like https://git.sapk.fr/Madeleine.js/examples/ajax.html

I think this would largely improve external rendering without introducing security risk by default but let the admin the possibility to load js or html that it trust.

[wxiaoguang]

Here it is: RENDER_CONTENT_MODE