Bring the theme browsing experience from 3.8 to the theme installer. First pass.

props matveb with assists from me and gcorne.
see #27055.

Built from https://develop.svn.wordpress.org/trunk@27499


git-svn-id: http://core.svn.wordpress.org/trunk@27341 1a063a9b-81f0-0310-95a4-ce76da25c4cd