MDL-86455 forms: disable all modal submit buttons on button click

To provide visual feedback and prevent multiple clicks
on no-submit buttons, all submit buttons within a modal
form will be disabled once any of them is clicked.

public/lib/form/amd/src/modalform.js

@@ -328,9 +328,13 @@ export default class ModalForm {
         formData = formData + '&' + encodeURIComponent(button.getAttribute('name')) + '=' +
             encodeURIComponent(button.getAttribute('value'));
 
+        this.disableButtons();
+
         const bodyContent = this.getBody(formData);
         this.modal.setBodyContent(bodyContent);
-        bodyContent.catch(Notification.exception);
+        bodyContent
+            .then(() => this.enableButtons())
+            .catch(Notification.exception);
     }
 
     /**
@@ -358,14 +362,16 @@ export default class ModalForm {
      * Disable buttons during form submission
      */
     disableButtons() {
-        this.modal.getFooter().find('[data-action]').attr('disabled', true);
+        this.modal.getModal()[0].querySelectorAll('input[type="submit"], [data-action]')
+            .forEach(el => el.setAttribute('disabled', true));
     }
 
     /**
      * Enable buttons after form submission (on validation error)
      */
     enableButtons() {
-        this.modal.getFooter().find('[data-action]').removeAttr('disabled');
+        this.modal.getModal()[0].querySelectorAll('input[type="submit"], [data-action]')
+            .forEach(el => el.removeAttribute('disabled'));
     }
 
     /**