Keep track of clicked button when submitting form

When submitting a form with a Stripe element in it, the stripe.js file (included with the module) submits the entire form with the form.submit() method. This does not preserve the button name which was clicked. If forms have multiple buttons (such as a Webform with both Back and Submit buttons), Drupal will find the first button in the form and act as though that button were clicked.

This could cause other big problems for forms that had File fields on the same form, since they will also end up being the resulting submit handler.