When uploads are stored on S3, by default Discourse will fetch the avatars and proxy them through to the requesting client. This is simple, but it can lead to significant inbound/outbound network load in the hosting environment.
This commit adds an optional redirect_avatar_requests GlobalSetting. When enabled, requests for user avatars will be redirected to the S3 asset instead of being proxied. This adds an extra round-trip for clients, but it should significantly reduce server load. To mitigate that extra round-trip for clients, a CDN with 'follow redirect' capability could be used.