Download filename for proxied images

[Nutomic]

When downloading a proxied image, it is always saved as image_proxy.webp. So if you download multiple images, they need to be renamed manually every time. It seems the Content-Disposition header can be set to change this:

• https://stackoverflow.com/a/3102276
• https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Content-Disposition

So when serving a proxied image, we should extract the filename from the url and put it in the Content-Disposition header. Relevant code: https://github.com/LemmyNet/lemmy/blob/main/crates/routes/src/images/download.rs#L42

Alternatively this could be implemented in pict-rs (cc @asonix).