$metadata->thumbnail_url is not guaranteed to be set
$metadata->thumbnail_url is not guaranteed to be set

We should probably have a separate class for this, so we can more
easily combine different technologies similar to oEmbed/OpenGraph.

foocorp/gnu-social,chimo/social,chimo/social,foocorp/gnu-social,chimo/social,foocorp/gnu-social,charnimda/lgbtpzn-social,charnimda/lgbtpzn-social,charnimda/lgbtpzn-social