Allow Authorization with Pundit Policy Namespacing

Fix https://github.com/thoughtbot/administrate/issues/1332

To use specific Policy Namespacing for administrate, like admin namespace policies for example, just define the pundit_namespaces in an array format, like below. It is also backward compatible with resolve_admin.

module Admin
  class ApplicationController < Administrate::ApplicationController
    include Administrate::Punditize
    def pundit_namespaces
      [:admin]
    end
  end
end