Themes: Hide 'Add New' with no JS. see #27055.

Built from https://develop.svn.wordpress.org/trunk@27936


git-svn-id: http://core.svn.wordpress.org/trunk@27766 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Theme Installer: Fix sorting, counts, keyboard navigation; add prev/next to previews.

props matveb.
see #27055.

Built from https://develop.svn.wordpress.org/trunk@27937


git-svn-id: http://core.svn.wordpress.org/trunk@27767 1a063a9b-81f0-0310-95a4-ce76da25c4cd