Background Updates: Spread them over the hour. Props Pento. Fixes #25833 for trunk 

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Twenty Fourteen: make sure styles for featured content on a static page are limited to posts within the main content area. Props iamtakashi, fixes #25938.
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Twenty Fourteen: remove unused class name, props obenland. See #25946.
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Twenty Fourteen: simplify and prefix post thumbnail sizes, props obenland. See #25946.
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Twenty Fourteen: first pass at adding help text to to "Help" tab for `edit.php` and `themes.php` screens. Props obenland for starter patch, see #25837.
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Twenty Fourteen: fix tabs to spaces in two files. See #25946.
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