directly return rustdoc rustdoc json instead of redirect

[syphar]

Until now, our rustdoc-json-download handlers just redirected to static.docs.rs, which points to S3 directly.

But with Fastly the invalidation of this service would be tricky and we would have to somehow trick S3 to return surrogate key headers to be able to properly invalidate. There are some workaround, but having the static. subdomain also clashes with the subdomain-per-crate idea.

So I decided to pull this back in. Since then this is now possible because we can stream the body directly from S3 to the client, and don't load the whole body into memory any more.