Add cron job to clean old activities

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/service/workers/clean_old_activity_worker.ex

@@ -0,0 +1,13 @@
+defmodule Mobilizon.Service.Workers.CleanOldActivityWorker do
+  @moduledoc """
+  Worker to clean old activity
+  """
+
+  use Oban.Worker, queue: "background"
+  alias Mobilizon.Service.CleanOldActivity
+
+  @impl Oban.Worker
+  def perform(%Job{}) do
+    CleanOldActivity.clean()
+  end
+end