GSettings can't be set with sudo from GDM's PostLogin on Wheezy (tried both with
"gsettings set" and "dconf write"), so we'll have to do this otherwise.

Then:

* Adapt the Unsafe Browser script accordingly: make it run
  tails-activate-winxp-theme as the clearnet user. This likely won't work
  either, for the same reason as why we're doing this entire commit at all,
  so likely this one will need more adaptation. We'll see.

* Don't try to delete /etc/skel/Desktop/*.desktop: it seems entirely useless
  (the users we need have already been created) and can't work now that we don't
  run this script as root anymore.