Fixed display bug on iPad when setting header video

【Contents of the fix】
■Top page > Fixed the problem that the alternative image is not displayed on iPad when selecting video or Youtube in the header content settings.
※With this fix, the recommended size of the alternative image when selecting videos and Youtube has been modified.

The following are the files that have been modified for this update.
【Corrected files】
admin/theme-options.php
footer.php
front-page.php
functions/head.php
header.php
languages/ja.mo
languages/ja.po
style.css

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