Theme installer:
 * Restore the feature filter.
 * Improve responsiveness.
 * Router updates, fixes.
 * Make "Upload Theme" button more consistent with the admin.
 * Avoid theme-count causing filters to jump.

props matveb.
see #27055.

Built from https://develop.svn.wordpress.org/trunk@27636


git-svn-id: http://core.svn.wordpress.org/trunk@27479 1a063a9b-81f0-0310-95a4-ce76da25c4cd