Multiple bug fixes

【Contents of the fix】
■Fixed a bug where the Hatena bookmark icon was not displayed on the share button in the fixed footer bar.
■Fixed a bug in the start tag of quick tags “Heading H4” and “Heading H5”.

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

【Modified file】
css/footer-bar.css
functions/custom_editor.php
style.css
template-parts/footer-bar.php

【Added files】
img/common/hatenabookmark-logomark.svg

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