Modify sort index for activities.

This prevents a problem that made the local timeline very slow when
there were few posts.

priv/repo/migrations/20180513104714_modify_activity_index.exs

@@ -0,0 +1,9 @@
+defmodule Pleroma.Repo.Migrations.ModifyActivityIndex do
+  use Ecto.Migration
+  @disable_ddl_transaction true
+
+  def change do
+    create index(:activities, ["id desc nulls last", "local"], concurrently: true)
+    drop_if_exists index(:activities, ["id desc nulls last"])
+  end
+end