Bug 18811: Add a script for checking authority data in hidden fields

Bug 18811: Add a script for checking authority data in hidden fields

If you edit an authority record while having data in hidden fields or
subfields, that data will be lost now.
This script can help you to unhide some fields and prevent data loss.

Test plan:
[1] Add a PERSO_NAME record. Fill e.g. 100b.
[2] Hide 100b in the PERSO_NAME framework.
[3] Run auth_show_hidden_data.pl and verify that it reports 100b in
    the PERSO_NAME framework.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>