Add rel="noopener" to target="_blank" links

The noopener construct protects from some nasty clickjacking attacks. We
can apply them savely to all our links since we don't rely on the
previously used page.

Some more details: https://mathiasbynens.github.io/rel-noopener/

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

public/js/extra.js

@@ -570,7 +570,9 @@ export function postProcess (code) {
     $(value).html(html)
   })
   // link should open in new window or tab
-  result.find('a:not([href^="#"]):not([target])').attr('target', '_blank')
+  // also add noopener to prevent clickjacking
+  // See details: https://mathiasbynens.github.io/rel-noopener/
+  result.find('a:not([href^="#"]):not([target])').attr('target', '_blank').attr('rel', 'noopener')
   // update continue line numbers
   const linenumberdivs = result.find('.gutter.linenumber').toArray()
   for (let i = 0; i < linenumberdivs.length; i++) {