Add make_unannounce_data helper function

lib/pleroma/web/activity_pub/utils.ex

@@ -278,6 +278,23 @@ def make_announce_data(
     if activity_id, do: Map.put(data, "id", activity_id), else: data
   end
 
+  @doc """
+  Make unannounce activity data for the given actor and object
+  """
+  def make_unannounce_data(
+        %User{ap_id: ap_id} = user,
+        %Object{data: %{"id" => id}} = object
+      ) do
+    %{
+      "type" => "Undo",
+      "actor" => ap_id,
+      "object" => id,
+      "to" => [user.follower_address, object.data["actor"]],
+      "cc" => ["https://www.w3.org/ns/activitystreams#Public"],
+      "context" => object.data["context"]
+    }
+  end
+
   def add_announce_to_object(%Activity{data: %{"actor" => actor}}, object) do
     with announcements <- [actor | object.data["announcements"] || []] |> Enum.uniq() do
       update_element_in_object("announcement", announcements, object)