Updating the #wp-auth-check login form to match the new admin design:

* Remove border-radius on the container and add a box shadow
* Style the top bar background to match the new #eee page background
* Replace the close button PNG with a Dashicon
* Remove a no-longer-needed button style override in buttons.css that caused padding to change when clicking the submit button.

See #25858.


Built from https://develop.svn.wordpress.org/trunk@26441


git-svn-id: http://core.svn.wordpress.org/trunk@26339 1a063a9b-81f0-0310-95a4-ce76da25c4cd