Auto-fill machine name using Name as 'source'

Still unsolved but totally makes sense. Let's use the code from other entity types that already solve this.

I think it might now be enough to use this form element type: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21...

Has already been partially fixed in 🐛 Token (& token type) machine name requirements RTBC

Let's see if this works.

The machine name field already works on TokenCustomTypeForm.php, but not on TokenCustomForm.php. @anybody suggested to me that he will take another look at this. For now, back to “Needs work”.