Add tracking of delay to streaming API

app/assets/javascripts/components/features/ui/containers/modal_container.jsx

@@ -123,7 +123,7 @@ const Modal = React.createClass({
     window.addEventListener('keyup', this._listener);
   },
 
-  componentDidUnmount () {
+  componentWillUnmount () {
     window.removeEventListener('keyup', this._listener);
   },
 

app/lib/feed_manager.rb

@@ -30,6 +30,7 @@ class FeedManager
   end
 
   def broadcast(timeline_id, options = {})
+    options[:queued_at] = (Time.now.to_f * 1000.0).to_i
     ActionCable.server.broadcast("timeline:#{timeline_id}", options)
   end