Replace unrar-free with unar

Unrar-free is working pretty badly and fails to open many .rar archives in Tails. It has tons of Debian bugs: https://bugs.debian.org/unrar-free.

Next thing to do: wait for Tails to be based on Jessie.

When we ship file-roller 3.6 or newer, we should stop installing unrar-free as unar will be used instead. Unfortunately, Wheezy will have 3.4, so that will be for Jessie.

See have archive manager support rar again (#5592 (closed)) for details.

Original created by @tails on 5838 (Redmine)