Allow the attachment endpoint to redirect to a CDN location

[quentez]

The current attachment endpoint requires authentication (at least for private posts). This makes the use of CDNs for attachments difficult since they usually don't play nice with authentication and things that can't be cached. A solution would be to perform access control on the attachment endpoint, return private posts directly, and redirect to a CDN for public posts.

Using a CDN seems like a must-have for things like large video posts that could potentially be accessed by a lot of people at the same time.

What I'm asking for is for the option to redirect if needed, self-hosters won't be using CDNs any time soon, but it's going to be needed by large public hosts pretty quickly.