Make choosing a header image from the media library play nicely with file...

Make choosing a header image from the media library play nicely with file replication plugins that do not guarantee images will be retained in the local filesystem.

* When passing an attachment ID to wp_crop_image(), use load_image_to_edit() to fetch the image via a url fopen when the image does not exist in the filesystem.
* Move load_image_to_edit() to wp-admin/includes/image.php so that it is always available for admin pages loads.
* Fallback to the height and width stored in the attachment meta when the image no longer exists in the filesystem.

see #19840


git-svn-id: https://core.svn.wordpress.org/trunk@20384 1a063a9b-81f0-0310-95a4-ce76da25c4cd