Twenty Twelve: Add permalink for image posts without a title

[obenland]

Appart from occasionally missing the date as described in #21721, Image posts may also miss a permalink to the single view, when they have no title.

An easy way to address this would be to wrap the post date in <time> markup and add the permalink, similar to how twentytwelve_entry_meta() does it.

[lancewillett]

Patch is a good start, but I think needs better escaping like esc_attr( get_the_time() ) in the title attribute.

[lancewillett]

In [21674]:

Twenty Twelve: add a better permalink markup structure to content-image.php template, for usability when image posts have no title. Props obenland, fixes #21723.