Merge branch 'fix/notification-id-index' into 'develop'

Add an `id desc nulls last` index to notifications

See merge request pleroma/pleroma!975

priv/repo/migrations/20190325185009_create_notification_id_index.exs

+defmodule Pleroma.Repo.Migrations.CreateNotificationIdIndex do
+  use Ecto.Migration
+
+  def change do
+  create index(:notifications, ["id desc nulls last"])
+  end
+end