Twenty Thirteen: fix non-responsive layout in IE8. Props phpmypython, fixes #24194.
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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.
Built from https://develop.svn.wordpress.org/trunk@30387


git-svn-id: http://core.svn.wordpress.org/trunk@30384 1a063a9b-81f0-0310-95a4-ce76da25c4cd