[FIX] website_event: unblock button when no reserve

In 11.0, this change e9454e79 solved the use case of:

- opening the registration of a ticket
- discard

=> the page must be reloaded to register a ticket

A new report is that since 9.0, if we try to register 0 ticket we would
also have to reload the page.

This commit backports e9454e79 and solves the 0 ticket registration.

10.0 version of 9.0's #24966
opw-1851622
closes #24991

addons/website_event/static/src/js/website_event.js

@@ -23,16 +23,23 @@ var EventRegistrationForm = Widget.extend({
         ev.preventDefault();
         ev.stopPropagation();
         var $form = $(ev.currentTarget).closest('form');
+        var $button = $(ev.currentTarget).closest('[type="submit"]');
         var post = {};
         $("#registration_form select").each(function() {
             post[$(this).attr('name')] = $(this).val();
         });
         return ajax.jsonRpc($form.attr('action'), 'call', post).then(function (modal) {
+            // Only needed for 9.0 up to saas-14
+            if (modal === false) {
+                $button.prop('disabled', false);
+                return;
+            }
             var $modal = $(modal);
             $modal.find('.modal-body > div').removeClass('container'); // retrocompatibility - REMOVE ME in master / saas-19
-            $modal.after($form).modal();
+            $modal.insertAfter($form).modal();
             $modal.on('click', '.js_goto_event', function () {
                 $modal.modal('hide');
+                $button.prop('disabled', false);
             });
         });
     },