Add tracking for YouTube videos #2156

This is a WIP. I'm tracking the following events right now

1. Load event for YouTube videos
2. Play/Paused/Buffering/Finished status of the video
3. Video Quality
4. Errors, reported by YouTube API
5. 10% increments of video played.
Should we enforce `&enablejsapi=1&origin=` in the content itself? Either via our [YouTube ckeditor plugin](https://github.com/mozilla/kuma/blob/master/media/js/libs/ckeditor/plugins/youtube/plugin.js#L144)? Or maybe we try to do this as a processing step in the python code? (cc @lmorchard)