Add header image uploads with cropping to the customizer.

props mcsf, ehg, gcorne.
see #21785.


git-svn-id: https://develop.svn.wordpress.org/trunk@27497 602fd350-edb4-49c9-b593-d223f7449a82
JS linting for [27497].

see #21785.


git-svn-id: https://develop.svn.wordpress.org/trunk@27498 602fd350-edb4-49c9-b593-d223f7449a82