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.


git-svn-id: https://develop.svn.wordpress.org/trunk@27636 602fd350-edb4-49c9-b593-d223f7449a82