Make the content of /etc/skel non-world-readable.

Otherwise, such files may be copied to /home/amnesia, and in turn to the
persistent volume, with unsafe permissions. That's no big deal in /home/amnesia
(that is itself not world-readable), *but* the root of the persistent volume has
to be world-readable.
Drop chmod that we're overriding later in config/chroot_local-hooks/99-permissions.
Have activate_custom_mounts create new directories with safe permissions.

This is the second part of the fix for #7443.
Set strict permissions on /home/amnesia (Closes: #7463)
Fix permissions on persistent directories that were created with unsafe permissions (Closes: #7458).
Automatically test persistent directories permissions (Closes: #7560)

These are primarily regression tests to prevent #7443 from coming back.
At the same time, we're testing the migration code that addresses #7458.
Bump pining value for wheezy-backports' shared-mime-info.

The idea is to override the custom 1.2 + upstream cherry-pick backport
we have in our APT repository. This commit shall be reverted when merging this
branch and removing that custom package.
Fix step name in Cucumber feature.
Fix expected permissions on persistent directories.
Remove files as the parent directory's owner.

Else, it can't possibly succeed.
The nm-system-connections directory is expected to have mode 755.
Fix name of the persistent CUPS directory.
The CUPS configuration directory also is expected to have mode 755.
Use read-write persistence when testing upgraded USB installations.

Otherwise e.g. the permission fixes won't get applied, and the
subsequent steps testing the permissions will fail.
Provide a source-destination mapping for persistent mounts.

Knowing that relationship makes it possible to do the permission
checks in the "persistent directories have safe access rights" step
without exceptions for each destination not in /home/$LIVE_USER.
Actually check that the ISO's Tails is installed.

The step "Tails is installed on USB drive $TARGET" only checks that
the *running* Tails is installed on $TARGET, which obviously fails
when doing an upgrade from ISO running an old Tails. That it worked
for the same scenario running the current Tails is just incidental.
Fix typo.
Unmount ISO when done with it.
Merge branch 'bugfix/7443-persistent-files-permission' into testing
Merge branch 'bugfix/7345-upgrade-from-iso-from-1.0-to-1.1' into testing