About page: Use box-sizing: border-box on the password meter so pixels line up.

Also eliminates a duplicate width: 100% property.

fixes #25603.

Built from https://develop.svn.wordpress.org/trunk@25845


git-svn-id: http://core.svn.wordpress.org/trunk@25757 1a063a9b-81f0-0310-95a4-ce76da25c4cd