Bug 24870: Translate installer data label

Bug 24870: Translate installer data label

This patch enable the translation of the
labels 'Mandatory' or 'Optional' present at install time
for groups of installer files.

To test:
1) Try install using de-DE language,
   at step3 verify 'Mandatory' and 'Optional'
   labels for marc21 or other data checklist
   Do not close this window!
2) Apply this patch
3) Update de-DE translation files, reinstall lang files
   cd misc/translator
   ./translate update de-DE
   ./translate install de-DE
4) Reload page at step3, now 'Mandatory' label
is translated to 'Pflichtfeld'
5) Verify strings in translation file
Search for
^msgid \"Mandatory\"
^msgid \"Optional\"
in po/de-DE-staff-prog.po.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Joy Nelson <joy@bywatersolutions.com>