This renames the file containing the `Custom_Image_Header` class to conform to the coding standards.
This commit also includes:
- A new `custom-header.php` that includes the new file, for anyone that may've been including the file directly.
- Replaces references to the old filename with the new filename.
See #47632.
Built from https://develop.svn.wordpress.org/trunk@45654
git-svn-id: http://core.svn.wordpress.org/trunk@45465 1a063a9b-81f0-0310-95a4-ce76da25c4cd
