Force browsers to re-validate cache before reuse

Upon new release, NGINX would return 200 OK with new static files.
Otherwise, NGINX would return 304 Not Modified with empty response, and browser continue to reuse cached static files.

(cherry picked from commit 4506a5e4901299dfcb8ba2524e2a6c451fffcf90)

server/nginx.conf

@@ -22,7 +22,7 @@ http {
     #tcp_nopush     on;
 
     keepalive_timeout  65;
-    expires 86400;
+    add_header Cache-Control "no-cache";
     etag on;
 
     include /etc/nginx/conf.d/*.conf;