Add header image uploads with cropping to the customizer

[nacin]

See #21355 for an explanation for why header image uploads (sans cropping) was removed from the 3.4 branch. This ticket is about adding it back, with a crop step, to ensure proper support.

Related: #21820

I was looking at the code on the Custom Header, the usage of steps on this page feels wrong;

We have the power to put all this stuff at the Media Box, in a way that the user have the same experience of cropping that he would have if he was inserting an image to the post.

Even if this is not the approach we take, at least the steps should have a POST then GET, at every step, on the second step you get a duplicate of your image if you reload the page, then in the third step you will get a duplicated crop.

I would love to see this page rebuild from scratch, maintaining backwards compatibility but still rebuilt.

Not saying that this should be on 3.5 but...