Card Details Popup, every "Sub-Popup" must use .back() instead of .close()

- .close() closes the popup. Until now it was ok, but as the Card
  Details are opened in a popup too, .back() must be used to get the
  same behaviour back
- .back() is also .close() if there is no "popup layer" left

client/components/activities/activities.js

@@ -269,7 +269,7 @@ Template.addReactionPopup.events({
       const cardComment = CardComments.findOne({_id: commentId});
       cardComment.toggleReaction(codepoint);
     }
-    Popup.close();
+    Popup.back();
   },
 })