Twenty Fourteen: move all controls for managing Featured Content to the Customizer. Also implement smart fallbacks to not using a custom tag with the "featured" tag and sticky posts. Props obenland and celloexpressions, see #25549.

git-svn-id: https://develop.svn.wordpress.org/trunk@26037 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fourteen: add contextual help text to Background and Colors controls in Customizer, using the `description` field. Props rachelbaker, see #25869.

git-svn-id: https://develop.svn.wordpress.org/trunk@26038 602fd350-edb4-49c9-b593-d223f7449a82