Twenty Fourteen: improve post thumbnail HTML output.

 * Add `aria-hidden` attribute to reduce verbosity on archive pages.
 * Add alt text in archives to avoid confusing link texts, see #30076 for context in Twenty Fifteen.

Props hiwhatsup, joedolson. Fixes #30144.

git-svn-id: https://develop.svn.wordpress.org/trunk@30387 602fd350-edb4-49c9-b593-d223f7449a82