Fixed several bugs and added RSS feed content setting function

【Contents of the fix】
■Fixed a bug in which the slider on the product page was not displayed when the sidebar was set to the left column in smartphone view.
■Fixed a bug where long URLs would sometimes be overflowed in the “URL” field of product pages.
■Added the ability to set the content of RSS feeds.

The following are the files that have been modified for this update.
【Corrected files】
・admin/theme-options.php
・admin/ink/footer.php
・admin/ink/header.php
・css/responsive.css
・language/ja.po
・language/ja.mo
・functions.php
・footer.php
・header.php
・style.css

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