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 in which YouTube videos do not play in a loop when the number of sliders is one.
■Fixed the bug that Google Analytics tracking code is not reflected in the custom script.

The following are the files that have been modified for this update.
【Modified file】
admin/inc/basic.php
comments.php
functions.php
functions/custom_css.php
functions/custom_script.php
functions/head.php
functions/seo.php
header.php
languages/ja.mo
languages/ja.po
style.css