Narrow the scope of the included wp-admin files loaded for `wp_generate_attachment_metadata()`, `wp_handle_upload()`, `wp_tempnam()`, and `wp_handle_sideload()`. Requires only `wp-admin/includes/file.php` and `wp-admin/includes/image.php` instead of `wp-admin/includes/admin.php`.
Props ocean90, lonelyvegan, soulseekah, pratikthink.
Merges [43589], [43604] to the 5.0 branch.
Fixes#43757.
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git-svn-id: http://core.svn.wordpress.org/branches/5.0@43602 1a063a9b-81f0-0310-95a4-ce76da25c4cd