feat(api): expose a way to submit feedback

Spot will need a way to submit call feedback using the iframe
api. For now expose a method on conference.js to submit that
feedback. Exposing on conference.js looks to be the existing
pattern... Also add an event to notify consumers of the iframe
api that feedback was submitted, as postMessage is async
and the notification can at least give some guarantee maybe.

I haven't updated documentation yet as I'm not confident
about this api.

modules/API/external/external_api.js

@@ -21,6 +21,7 @@ const commands = {
     displayName: 'display-name',
     email: 'email',
     hangup: 'video-hangup',
+    submitFeedback: 'submit-feedback',
     toggleAudio: 'toggle-audio',
     toggleChat: 'toggle-chat',
     toggleContactList: 'toggle-contact-list',
@@ -38,6 +39,7 @@ const events = {
     'audio-availability-changed': 'audioAvailabilityChanged',
     'audio-mute-status-changed': 'audioMuteStatusChanged',
     'display-name-change': 'displayNameChange',
+    'feedback-submitted': 'feedbackSubmitted',
     'incoming-message': 'incomingMessage',
     'outgoing-message': 'outgoingMessage',
     'participant-joined': 'participantJoined',