Add header image uploads with cropping to the customizer.

props mcsf, ehg, gcorne.
see #21785.

Built from https://develop.svn.wordpress.org/trunk@27497


git-svn-id: http://core.svn.wordpress.org/trunk@27339 1a063a9b-81f0-0310-95a4-ce76da25c4cd
JS linting for [27497].

see #21785.

Built from https://develop.svn.wordpress.org/trunk@27498


git-svn-id: http://core.svn.wordpress.org/trunk@27340 1a063a9b-81f0-0310-95a4-ce76da25c4cd