Only show "Add New Theme" block on Manage Themes screen if not Multisite and the user can install themes.

props markoheijnen.
see #25948.
Built from https://develop.svn.wordpress.org/trunk@26288


git-svn-id: http://core.svn.wordpress.org/trunk@26193 1a063a9b-81f0-0310-95a4-ce76da25c4cd