Add `X-Cache-Status` HTTP header

Purpose: Would be nice there was a simple way to check if caching is actually functional.

In discourse/nginx.sample.conf at main · discourse/discourse · GitHub for the frontend:

add_header X-Cache-Status $upstream_cache_status;

Quote Module ngx_http_upstream_module

$upstream_cache_status
keeps the status of accessing a response cache (0.8.3). The status can be either “MISS”, “BYPASS”, “EXPIRED”, “STALE”, “UPDATING”, “REVALIDATED”, or “HIT”.