add migration

db/migrate/20190611130900_add_maintenance_mode_to_users.rb

@@ -0,0 +1,12 @@
+require 'carto/db/migration_helper'
+
+include Carto::Db::MigrationHelper
+
+migration(
+  Proc.new do
+    add_column :users, :maintenance_mode, :boolean, null: false, default: false
+  end,
+  Proc.new do
+    drop_column :users, :maintenance_mode
+  end
+)