Infinite scroll for themes.php and theme-install.php. Bump per page limit for...

Infinite scroll for themes.php and theme-install.php. Bump per page limit for themes.php to 999. Props helenyhou, DH-Shredder. see #19815

git-svn-id: https://core.svn.wordpress.org/trunk@19887 1a063a9b-81f0-0310-95a4-ce76da25c4cd