[FIX] website: submit form

Example where the issue occurs:
- Publish an event
- Go to the website, register for that event
- At confirmation, when the email address is requested, click quickly
  several times on the 'Continue' button

Several registrations are created since nothing prevents multi-clicks.

The stateful button feature of the `button.js` jQuery plugin has been
dropped in Bootstrap 4 (see [1]).

[1] https://getbootstrap.com/docs/4.1/migration/#buttons

opw-1958580

closes odoo/odoo#32571

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/website/static/src/js/content/website_root.js

@@ -282,8 +282,8 @@ var WebsiteRoot = BodyManager.extend({
         var $buttons = $(ev.currentTarget).find('button[type="submit"], a.a-submit');
         _.each($buttons, function (btn) {
             var $btn = $(btn);
-            $btn.attr('data-loading-text', '<i class="fa fa-spinner fa-spin"></i> ' + $(btn).text());
-            $btn.button('loading');
+            $btn.html('<i class="fa fa-spinner fa-spin"></i> ' + $btn.text());
+            $btn.prop('disabled', true);
         });
     },
     /**