puppet: Set aggressive caching headers on immutable webpack files.

A partial fix for #3470.

puppet/zulip/files/nginx/zulip-include-frontend/app

@@ -14,6 +14,11 @@ location /static/ {
 
     # Set a nonexistent path, so we just serve the nice Django 404 page.
     error_page 404 /django_static_404.html;
+
+    # These files are hashed and thus immutable; cache them aggressively.
+    location /static/webpack-bundles {
+        add_header Cache-Control "public, max-age=31536000, immutable";
+    }
 }
 
 # Send longpoll requests to Tornado