Second pass at infinite scroll for themes, including polling, fixed paging,...

Second pass at infinite scroll for themes, including polling, fixed paging, and fewer ajax calls. props DH-Shredder, helenyhou, garyc40. see #19815.

git-svn-id: https://core.svn.wordpress.org/trunk@19971 1a063a9b-81f0-0310-95a4-ce76da25c4cd