Multiple bug fixes

【Contents of the fix】
■【iPhone】Fixed an issue where the Hatena bookmark icon was not displayed on the share button in the fixed footer bar.
■SSL support for “Google Custom Search” widget.
■Fixed a bug in the display of Mega Menu A on the Welcart registration/login page.
■Changed the specifications of the navigation label display of the Mega Menu sub-item.
(The sub-item displayed in the mega menu has been changed from retrieving the category name and displaying it to reflecting the navigation label in Appearance > Menu.)

The following files have been modified for this update.

【Modified files】
css/footer-bar.css
functions/update_notifier.php
header.php
languages/ja.mo
languages/ja.po
style.css
template-parts/footer-bar.php
template-parts/megamenu.php
widget/google_search.php

【Added files】
img/hatenabookmark-logomark.svg