Use the best thumbnail/placeholder/cover image available

[paulirish]

I've captured the general problem here: https://github.com/paulirish/lite-youtube-embed/blob/master/youtube-thumbnail-urls.md

Two things we can do:

• Do the sddefault->hqdefault fallback
  • When doing this, apply referrerpolicy (Set referrerpolicy=origin on auto generated placeholders. ampproject/amphtml#3940)
• Consider using webp if supported, falling back to jpg

A few other bugs have touched on this mostly #54

[zachleat]

I don’t think it makes sense to include in this plugin code but I’ll just mention another option I use to see if it sparks anything—I use the OpenGraph image for the video URL! Some detail here https://www.zachleat.com/web/youtube-embeds/#higher-quality-posters