Avoid loading content from Youtube unless the reader clicks to view it

[gpaumier]

There are Youtube videos embedded in a few pages. At this time they're using the youtube-nocookie domain, so cookies aren't set until the reader plays the video, but some content is still loaded (e.g. thumbnail and controls). Ideally those would be fetched during the nikola compilation process (e.g. using micawber and then replaced by the video if and only if the reader clicks to play it.