migrations: add is_moderator index for users table

priv/repo/migrations/20180903114437_users_add_is_moderator_index.exs

+defmodule Pleroma.Repo.Migrations.UsersAddIsModeratorIndex do
+  use Ecto.Migration
+
+  def change do
+    create index(:users, ["(info->'is_moderator')"], name: :users_is_moderator_index, using: :gin)
+  end
+end