add sort index for activities

priv/repo/migrations/20171109141309_add_sort_index_to_activities.exs

+defmodule Pleroma.Repo.Migrations.AddSortIndexToActivities do
+  use Ecto.Migration
+  @disable_ddl_transaction true
+
+  def change do
+    create index(:activities, ["id desc nulls last"], concurrently: true)
+  end
+end