title: A way to mark notification as seen without reading the notification feed?
body: <p>My situation is the following:
  I'm using the stream-js library. I add entries to the notification feeds of users for certain events - comments, follows, etc. After I write to their feed I also send a push notification to that user's device. </p>
  
  <p>If a user clicks on a push notification I want to be able to mark the corresponding activity as seen. There's currently no way to do that since the  <code>add</code> or <code>addToMany</code> calls do not return the ids of the added activities for me to send in the notification payload.</p>
  
  <p>Ideally I'd want a way to mark a notification feed item as seen either by an activity group id or by some other unique id (or the <code>foreignId</code>). Is there a way to do that? If not, what is the alternative?</p>