Twenty Fourteen: Make featured content in Customizer contextual to the front page

[kdoran]

Featured Content options should only be available on the front page because they aren't used on other pages.

There is no active_callback capability on Customizer sections. For now we can just hide all the controls and could potentially update the help text to do that.

[celloexpressions]

Worked on this at the WCLA Contributor Day and got to a patch, but decided that we need to wait for contextual sections before we can actually implement this. While we can hide most of the UI now, it doesn't make sense with the section still displayed.

[celloexpressions]

Make featured content contextual to the front page.

[celloexpressions]

Let's show off this new feature (contextual sections and panels) in 4.1 by implementing it for Twenty Fourteen, which has a perfect use-case with featured content. See the similarly numbered #29758.

[westonruter]

commit +1

[lancewillett]

Tests out well. Nice change.

[lancewillett]

In 30143:

Twenty Fourteen: Make featured content in Customizer contextual to the front page.

Props celloexpressions, fixes #29578.