Add new action 'delete_user_form' in `wp-admin/users.php` and `wp-admin/network/users.php`.

Props SergeyBiryukov, usermrpapa.
Fixes #27230.

Built from https://develop.svn.wordpress.org/trunk@28885


git-svn-id: http://core.svn.wordpress.org/trunk@28684 1a063a9b-81f0-0310-95a4-ce76da25c4cd