Enable browser caching of mtc.js as described in https://github.com/mautic/mautic/issues/3656

browser-cache.patch

diff --git a/app/bundles/CoreBundle/Controller/JsController.php b/app/bundles/CoreBundle/Controller/JsController.php
index 1bd7fda30..a108d9e85 100644
--- a/app/bundles/CoreBundle/Controller/JsController.php
+++ b/app/bundles/CoreBundle/Controller/JsController.php
@@ -33,7 +33,7 @@ class JsController extends CommonController
             $dispatcher->dispatch(CoreEvents::BUILD_MAUTIC_JS, $event);
         }
 
-        return new Response($event->getJs(), 200, ['Content-Type' => 'application/javascript']);
+        return new Response($event->getJs(), 200, ['Content-Type' => 'application/javascript', 'Cache-Control' => 'public, max-age=7200']);
     }
 
     /**