Migrate to a good Content Security Policy (#1023)

So attacks such as XSS (see [0]) will no longer be of an issue.

[0]: https://github.com/omarroth/invidious/issues/1022

assets/js/embed.js

@@ -1,3 +1,5 @@
+var video_data = JSON.parse(document.getElementById('video_data').innerHTML);
+
 function get_playlist(plid, retries) {
     if (retries == undefined) retries = 5;