Fixed bugs and added functions

【Contents of the fix】
■【】Fixed a bug in the header content where the alternative image for smartphones was not displayed when YouTube was set.
■Fixed a bug that the animation in the title area does not work when MP4 or YouTube is selected in the header content.
■Added pager setting to display under archives in theme options.

【Fixed files】
admin/css/my_admin.css
admin/inc/blog.php
header.php
languages/ja.mo
languages/ja.po
style.css
template-parts/navigation.php
template-parts/navigation_home.php