refactor: use iframes for gist embedding instead of gist-embed

The used library gist-embed relies on GitHub Gist's JSONP
endpoint which is a risk for XSS injection. By adding untrusted
content from GitHub into the DOM it also follows very bad
practises. Using the iframe embedding has the disadvantage of
not having the proper height for the frame auto-loaded, but
the security benefits are worth it.

Signed-off-by: Erik Michelson <github@erik.michelson.eu>

public/js/htmlExport.js

@@ -22,4 +22,3 @@ const $ = require('jquery')
 window.jQuery = $
 window.$ = $
 require('bootstrap')
-require('gist-embed/gist-embed.min')