Eslint prefer-const (#17864)

* adds prefer-const to eslint config and runs fixer

* reverts unintended change

ui/app/instance-initializers/track-csp-event.js

@@ -1,5 +1,5 @@
 export function initialize(appInstance) {
-  let service = appInstance.lookup('service:csp-event');
+  const service = appInstance.lookup('service:csp-event');
   service.attach();
 }