Twenty Fourteen: code cleanup, props kovshenin. See #25946.

 * Filtering `twentyfourteen_has_featured_posts()` false suggests the filter expects a boolean value, but we expect an array. Use an existing function instead.
 * In both style.css and ie.css use Genericons, so let's not forget to declare the dependancy.
 * Make sure Lato font is enqueued in `admin_print_scripts`.
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