Twenty Twelve: use the time element for image attachment views, and...

Twenty Twelve: use the time element for image attachment views, and `esc_html()` for `get_the_date()` in the element's output. Props iandstewart, see #19978.


git-svn-id: https://core.svn.wordpress.org/trunk@20003 1a063a9b-81f0-0310-95a4-ce76da25c4cd