Improve default public file server cache-control further

Set `immutable` for digested assets, this improve modern browsers
behavior, saves them from ever revalidating.

Set `stale-while-revalidate` for non-digested files. This allows
caching proxies to ever only revalidate at most once per minute
and asyncrhonously.

Author: byroot

railties/lib/rails/generators/rails/app/templates/config/environments/production.rb.tt

@@ -22,9 +22,12 @@ Rails.application.configure do
   config.public_file_server.headers = {
     "cache-control" => lambda do |path, _|
       if path.start_with?("/assets/")
-        "public, max-age=#{1.year.to_i}"
+        # Files in /assets/ are expected to be fully immutable.
+        # If the content change the URL too.
+        "public, immutable, max-age=#{1.year.to_i}"
       else
-        "public, max-age=#{1.minute.to_i}"
+        # For anything else we cache for 1 minute.
+        "public, max-age=#{1.minute.to_i}, stale-while-revalidate=#{5.minutes.to_i}"
       end
     end
   }