Multiple bug fixes / support for WP5.8 widget block editor

【Contents of the fix】
■Support for the WordPress 5.8 widget block editor
■Fixed a bug in WP5.8 that the setting field is not displayed after changing the fixed page template when using the block editor.
■Supports the “Contact Form 7 Multi-Step Forms” plug-in.
■Fixed a bug that caused a blank space to appear below the header content.

【Modified files】
admin/js/my_script.js
css/design-plus.css
functions/head.php
languages/ja.mo
languages/ja.po
functions.php
style.css