Populate thumbnails with video preview, where we do not yet have screenshots

[glennlunder]

trafficstars

When we add a Youtube link, we automatically gain a preview image, that could be used as the basis for a thumbnail for the production. This would help with avoiding a situation like in this image;

Here, it looks like most of our demo entries are just empty placeholders, and users are way less likely to click those than if there was an image there. This goes some way to remedying that.

[psenough]

was about to add this issue to github. this is definitely worth having. there are some default youtube thumbnail urls that can be used to grab the thumbnail used on youtube directly.

https://img.youtube.com/vi/<youtubeid>/maxresdefault.jpg

although they are not always available for all youtube videos.