activitypub: prime OGP crawler cache when new messages are inserted into the database

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -88,6 +88,10 @@ def insert(map, local \\ true) when is_map(map) do
           recipients: recipients
         })
 
+      Task.start(fn ->
+        Pleroma.Web.RichMedia.Helpers.fetch_data_for_activity(activity)
+      end)
+
       Notification.create_notifications(activity)
       stream_out(activity)
       {:ok, activity}