activity: add helper to fetch an activity's parent

lib/pleroma/activity.ex

@@ -82,4 +82,10 @@ defmodule Pleroma.Activity do
   def normalize(obj) when is_map(obj), do: Activity.get_by_ap_id(obj["id"])
   def normalize(ap_id) when is_binary(ap_id), do: Activity.get_by_ap_id(ap_id)
   def normalize(_), do: nil
+
+  def get_in_reply_to_activity(%Activity{data: %{"object" => %{"inReplyTo" => ap_id}}}) do
+    get_create_activity_by_object_ap_id(ap_id)
+  end
+
+  def get_in_reply_to_activity(_), do: nil
 end