Restore the staticRoot option from older versions of Yesod's scaffolding, so I can host static files on a subdomain easily

src/Foundation.hs

@@ -77,6 +77,16 @@ instance Yesod App where
         120    -- timeout in minutes
         "config/client_session_key.aes"
 
+    -- Redirect static requests to a subdomain - this is recommended for best
+    -- performance, since serving static files does not need your session
+    -- cookies and they can be served from the frontend HTTP server without
+    -- hitting the app server at all.
+    -- https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Serve-static-files-from-a-separate-domain.md
+    urlParamRenderOverride app (StaticR s) _ = do
+      staticRoot <- appStaticRoot . appSettings $ app
+      return . uncurry (joinPath app staticRoot) . renderRoute $ s
+    urlParamRenderOverride _ _ _ = Nothing
+
     -- Yesod Middleware allows you to run code before and after each handler function.
     -- The defaultYesodMiddleware adds the response header "Vary: Accept, Accept-Language" and performs authorization checks.
     -- Some users may also want to add the defaultCsrfMiddleware, which: