feat(welcome-page): Add viewed analytics event.

react/features/welcome/components/AbstractWelcomePage.js

      */
     componentDidMount() {
         this._mounted = true;
+        sendAnalytics(createWelcomePageEvent('viewed', undefined, { value: 1 }));
     }
 
     /**

react/features/welcome/components/WelcomePage.web.js

      * @returns {void}
      */
     componentDidMount() {
+        super.componentDidMount();
+
         document.body.classList.add('welcome-page');
         document.title = interfaceConfig.APP_NAME;