Twenty Fifteen: Simplify the header, sidebar, background controls and make customization faster for users to do. This has the added benefit of fixing our bug where hidden header text wasn't being updated on color scheme switch. Nice.

Props celloexpressions, iamtakashi, fixes #30164 and #29980.


git-svn-id: https://develop.svn.wordpress.org/trunk@30230 602fd350-edb4-49c9-b593-d223f7449a82