Added functions / fixed multiple bugs

【Contents of the fix】
■Changed specifications to allow custom CSS, custom scripts, and SEO settings (meta tags) for individual posts.
■Fixed a bug that product images on the product category archive page were not displayed on mobile devices when TCD Theme > Site Preferences > Thumbnail Hover Effect was set to “Slide”.
■Fixed a bug that the width of the top page “Free Space” content does not become 100% even if the width is set to “100% width size” on mobile devices.
■Updated the page builder to support WordPress 5.5.
■Fixed a minor bug.

The following are the files that have been modified for this update.
【Modified file】
Mcomments.php
css/responsive.css
functions.php
functions/custom_css.php
functions/custom_script.php
functions/head.php
functions/seo.php
functions/update_notifier.php
languages/ja.mo
languages/ja.po
pagebuilder/assets/admin/css/admin.css
pagebuilder/assets/admin/js/admin.js
pagebuilder/assets/admin/js/editor.js
pagebuilder/assets/admin/js/repeater.js
pagebuilder/assets/admin/js/slider.js
pagebuilder/modules/slider.php
pagebuilder/modules/tab.php
pagebuilder/pagebuilder.php
style.css

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