Modification of wording / WP5.0 support

【Corrections】
■Support for WordPress 5.0 (Gutenberg)
■Fixed an error in the wording of the navigation at the bottom of the article details page when viewed from a smartphone.
■Deleted unnecessary files.

The following are the files that have been modified for this update.
【Fixed files】
functions/next_prev.php
languages/ja.mo
languages/ja.po
pagebuilder/assets/admin/css/admin.css
pagebuilder/assets/admin/js/admin.js
pagebuilder/assets/admin/js/googlemap.js
pagebuilder/assets/css/googlemap.css
pagebuilder/assets/css/pagebuilder.css
pagebuilder/assets/css/simple_table.css
pagebuilder/assets/css/slider.css
pagebuilder/assets/js/googlemap.js
pagebuilder/modules/catchcopy.php
pagebuilder/modules/googlemap.php
pagebuilder/modules/headline.php
pagebuilder/modules/image.php
pagebuilder/modules/simple_table.php
pagebuilder/modules/slider.php
pagebuilder/modules/tab.php
pagebuilder/pagebuilder.php
style.css

【Deleted files】
pagebuilder/assets/css/staff_list.css
pagebuilder/modules/staff_list.php