Use remote conversation URI info to stitch convos together

If we know the URI sent from the remote party, and we don't know the
notice it is replying to, we might still be able to put it in the same
conversation thread!

plugins/OStatus/classes/Ostatus_profile.php

@@ -684,6 +684,10 @@ class Ostatus_profile extends Managed_DataObject
                     $options['reply_to'] = $orig->id;
                 }
             }
+            if (!empty($activity->context->conversation)) {
+                // we store the URI here, Notice class can look it up later
+                $options['conversation'] = $activity->context->conversation;
+            }
 
             $location = $activity->context->location;
             if ($location) {