Update the ISO verification instructions using GNOME

Originally created by @sajolida on #7162 (Redmine)

In our ISO verification instructions for GNOME we advertise the use of
OpenPGP through seahorse-plugins, but that used to be broken on recent
Debian and Ubuntu.

intrigeri reported that:

• It works fine in testing/sid.
• For Wheezy, one needs to install seahorse-nautilus + shared-mime-info
  from the backports, then it works just fine.

Status for Ubuntu should be investigated and updated.

Feature Branch: doc/7162-gnome_verif

Related issues

• Related to #8580 (closed)
• Related to #6851 (closed)