feat(external API): add feedbackPromptDisplayed event

Adds a new event fired when Jitsi Meet shows the feedback prompt.

modules/API/API.js

         this._sendEvent({ name: 'feedback-submitted' });
     }
 
+    /**
+     * Notify external application (if API is enabled) that the feedback prompt
+     * has been displayed.
+     *
+     * @returns {void}
+     */
+    notifyFeedbackPromptDisplayed() {
+        this._sendEvent({ name: 'feedback-prompt-displayed' });
+    }
+
     /**
      * Notify external application (if API is enabled) that the screen sharing
      * has been turned on/off.

modules/API/external/external_api.js

     'display-name-change': 'displayNameChange',
     'email-change': 'emailChange',
     'feedback-submitted': 'feedbackSubmitted',
+    'feedback-prompt-displayed': 'feedbackPromptDisplayed',
     'incoming-message': 'incomingMessage',
     'outgoing-message': 'outgoingMessage',
     'participant-joined': 'participantJoined',

react/features/feedback/components/FeedbackDialog.web.js

 
 import { cancelFeedback, submitFeedback } from '../actions';
 
+declare var APP: Object;
 declare var interfaceConfig: Object;
 
 const scoreAnimationClass
      */
     componentDidMount() {
         sendAnalytics(createFeedbackOpenEvent());
+        if (typeof APP !== 'undefined') {
+            APP.API.notifyFeedbackPromptDisplayed();
+        }
     }
 
     /**