Preview of menu machine name is inaccurate while adding a menu

This corresponds to Drupal 7 issue: https://www.drupal.org/node/2021571

The custom menu add form has an element to show the machine name while the title is being typed. After saving, that machine name is not editable, but is shown on the edit menu form.

During creation (add), if the menu title is "hello," it shows the machine name will be hello. That's not what happens, though. The machine name is actually "menu-hello", and "menu-hello" is what is shown after saving.