Fixed Bugs/Fixes related to SNS buttons

■【iPhone】Fixed a bug where the Hatena bookmark icon was not displayed on the share button in the fixed footer bar.
■Removed the discontinued Google+ service from the social networking buttons in the header and footer and replaced it with YouTube.
■Removed discontinued Google+ from the contributor profile.

The following are the files that have been modified for this update.

【Modified files】
admin/inc/basic.php
admin/theme-options.php
author.php
css/footer-bar.css
footer.php
functions/update_notifier.php
functions/user_profile.php
header.php
languages/ja.mo
languages/ja.po
style.css
template-parts/footer-bar.php

【Added files】
img/hatenabookmark-logomark.svg