Include room session id in "joined" events.

This helps with matching signaling sessions with sessions in Talk.

room.go

@@ -439,6 +439,9 @@ func (r *Room) PublishSessionJoined(session Session, sessionData *RoomSessionDat
 			},
 		},
 	}
+	if session, ok := session.(*ClientSession); ok {
+		message.Event.Join[0].RoomSessionId = session.RoomSessionId()
+	}
 	if err := r.publish(message); err != nil {
 		log.Printf("Could not publish session joined message in room %s: %s", r.Id(), err)
 	}