Add Oxford comma to password hint.

props trepmal.
fixes #26457.
Built from https://develop.svn.wordpress.org/trunk@27246


git-svn-id: http://core.svn.wordpress.org/trunk@27103 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix theme preview URL for themes loaded via AJAX on Install Themes screen.

props ocean90.
fixes #27128.
Built from https://develop.svn.wordpress.org/trunk@27247


git-svn-id: http://core.svn.wordpress.org/trunk@27104 1a063a9b-81f0-0310-95a4-ce76da25c4cd