disable-form-button.php

<?php

// add a spinner to the upload button when it is clicked
$this->registerJs(<<<JS
$('#import-form').on('beforeSubmit', function() {
    var button = $(this).find('#import-submit-button');
    button.prop('disabled', true);
    button.append(' <i class="fa fa-spinner fa-spin"></i>');
});
JS
);