feat: expose MEETING_ID_SET event (#1484)

* expose meeting-id-set event

* typo

JitsiConferenceEventManager.js

     this.chatRoomForwarder.forward(XMPPEvents.MUC_JOINED,
         JitsiConferenceEvents.CONFERENCE_JOINED);
 
+    this.chatRoomForwarder.forward(XMPPEvents.MEETING_ID_SET,
+        JitsiConferenceEvents.CONFERENCE_UNIQUE_ID_SET);
+
     // send some analytics events
     chatRoom.addListener(XMPPEvents.MUC_JOINED,
         () => {

JitsiConferenceEvents.js

  */
 export const CONFERENCE_LEFT = 'conference.left';
 
+/**
+ * Indicates that the conference unique identifier has been set.
+ */
+export const CONFERENCE_UNIQUE_ID_SET = 'conference.unique_id_set';
+
 /**
  * Indicates that the connection to the conference has been established
  * XXX This is currently fired whenVthe *ICE* connection enters 'connected'

doc/API.md

         - `LAST_N_ENDPOINTS_CHANGED` - last n set was changed (parameters - leavingEndpointIds(array) ids of users leaving lastN, enteringEndpointIds(array) ids of users entering lastN)
         - `CONFERENCE_JOINED` - notifies the local user that he joined the conference successfully. (no parameters)
         - `CONFERENCE_LEFT` - notifies the local user that he left the conference successfully. (no parameters)
+        - `CONFERENCE_UNIQUE_ID_SET` - notifies the local user that the unique id for a meeting has been set. (parameters - meetingId(string))
         - `DTMF_SUPPORT_CHANGED` - notifies if at least one user supports DTMF. (parameters - supports(boolean))
         - `USER_ROLE_CHANGED` - notifies that role of some user changed. (parameters - id(string), role(string))
         - `USER_STATUS_CHANGED` - notifies that status of some user changed. (parameters - id(string), status(string))