Customizer: Enhance Background Image/Upload controls.

* Adopt the Header Image layout to the Background Image/Upload control.
* Remove templates for interactive audio/video previews. They haven't worked yet.
* Don't show a "Remove" button when the default value is set.
* Avoid calculating of container/placeholder heights which are unrelated to custom headers.

see #21483.

git-svn-id: https://develop.svn.wordpress.org/trunk@30712 602fd350-edb4-49c9-b593-d223f7449a82