Add a subgroup_creation_level column to the namespaces table

db/migrate/20190626175626_add_group_creation_level_to_namespaces.rb

+# frozen_string_literal: true
+
+class AddGroupCreationLevelToNamespaces < ActiveRecord::Migration[5.1]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  disable_ddl_transaction!
+  
+  def up
+    unless column_exists?(:namespaces, :subgroup_creation_level)
+      add_column_with_default(:namespaces, :subgroup_creation_level, :integer, default: 0)
+    end
+  end
+
+  def down
+    if column_exists?(:namespaces, :subgroup_creation_level)
+      remove_column(:namespaces, :subgroup_creation_level)
+    end
+  end
+end