Reduce the dependencies of the Web ExternalAPI

We broke external_api.min.js by importing react/features/util which
imported react/features/base/lib-jitsi-meet.

1. To reduce the risks of such a breakage until we add
   external_api.min.js to the torture tests, import as little as
   possible in modules/API/external/external_api.js.
2. Use the global JitsiMeetJS on Web in react/features/base/util.

modules/API/external/external_api.js

 import EventEmitter from 'events';
 
-import { urlObjectToString } from '../../../react/features/base/util';
+import { urlObjectToString } from '../../../react/features/base/util/uri.js';
 import {
     PostMessageTransportBackend,
     Transport

react/features/base/util/loadScript.web.js

-import JitsiMeetJS from '../lib-jitsi-meet';
+/* @flow */
+
+declare var JitsiMeetJS: Object;
 
 /**
  * Loads a script from a specific URL. The script will be interpreted upon load.
  * @returns {Promise} Resolved with no arguments when the script is loaded and
  * rejected with the error from JitsiMeetJS.ScriptUtil.loadScript method.
  */
-export function loadScript(url) {
+export function loadScript(url: string) {
     return new Promise((resolve, reject) =>
         JitsiMeetJS.util.ScriptUtil.loadScript(
             url,
             /* async */ true,
             /* prepend */ false,
             /* relativeURL */ false,
-            /* loadCallback */ () => resolve(),
-            /* errorCallback */ error => reject(error)));
+            /* loadCallback */ resolve,
+            /* errorCallback */ reject));
 }