Add activity expiration worker

This is a worker that runs every minute and deletes expired activities.
It's based heavily on the scheduled activities worker.

test/activity_expiration_worker_test.exs

@@ -0,0 +1,17 @@
+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2018 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.ActivityExpirationWorkerTest do
+  use Pleroma.DataCase
+  alias Pleroma.Activity
+  import Pleroma.Factory
+
+  test "deletes an activity" do
+    activity = insert(:note_activity)
+    expiration = insert(:expiration_in_the_past, %{activity_id: activity.id})
+    Pleroma.ActivityExpirationWorker.perform(:execute, expiration.id)
+
+    refute Repo.get(Activity, activity.id)
+  end
+end