Previously, the image for the thumbnail was taken from the oembed endpoint.
Since these images are not available for some unlisted videos,
the thumbnails are now sourced from Open Graph instead.
When we "pull hotlinked images" on onebox images, they are added to the uploads table and their dominant color is calculated. This commit adds the data to the HTML so that it can be used by the client in the same way as non-onebox images. It also adds specific handling to the new `discourse-lazy-videos` plugin.
