Cache onebox images and/or serve them from the main domain

[Falco]

@kuba-orlik

It’s hard to achieve compliance with Discourse, as the Onebox feature makes the browser fetch the thumbnail from the original website, making it a third-party request

As said by @Firepup650 above, this is not the case at all, as Discourse automatically downloads and serves Oneboxed images, which is even a default setting.

You can even make this more strict by toggling block hotlinked media.