PhantomJS: Fix rendering of panels using Prometheus datasource

In 043bb59 a URLSearchParams usage was introduced which is not supported by PhantomJS. @babel/polyfill(deprecated) does not contain polyfill for URLSearchParams, hence the code (and Prometheus graphs rendering) was failing in PhantomJS environment.

The solution is to add https://www.npmjs.com/package/url-search-params-polyfill that takes care of the URLSearchParams

public/app/app.ts

@@ -1,6 +1,7 @@
 import '@babel/polyfill';
 import 'whatwg-fetch'; // fetch polyfill needed for PhantomJs rendering
 import 'abortcontroller-polyfill/dist/polyfill-patch-fetch'; // fetch polyfill needed for PhantomJs rendering
+import 'url-search-params-polyfill'; // fetch polyfill needed for PhantomJs rendering
 import 'file-saver';
 import 'lodash';
 import 'jquery';