activity: add normalize() to find a complete activity given either URI or partial structure

lib/pleroma/activity.ex

@@ -78,4 +78,8 @@ def get_create_activity_by_object_ap_id(ap_id) when is_binary(ap_id) do
   end
 
   def get_create_activity_by_object_ap_id(_), do: nil
+
+  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
 end