Add activity expirations table

Add a table to store activity expirations. An activity can have zero or
one expirations. The expiration has a scheduled_at field which stores
the time at which the activity should expire and be deleted.

test/activity_test.exs

@@ -164,4 +164,13 @@ defmodule Pleroma.ActivityTest do
       Pleroma.Config.put([:instance, :limit_to_local_content], :unauthenticated)
     end
   end
+
+  test "add an activity with an expiration" do
+    activity = insert(:note_activity)
+    insert(:expiration_in_the_future, %{activity_id: activity.id})
+
+    Pleroma.ActivityExpiration
+    |> where([a], a.activity_id == ^activity.id)
+    |> Repo.one!()
+  end
 end