Bug 10942: Provide a way for package upgrades to update template translations

This patch adds a new config variable AUTOMATIC_TRANSLATIONS_UPDATE at
/etc/default/koha-common that is used to control whether the upgrade
process should trigger a

 $ koha-translate --update <lang_code>

command for each installed template translation language.

To test:
- Have a koha-common setup with some languages installed
  (e.g. koha-translate --install es-ES)
- Apply the patch and build a package for it.
- Install it.
- A new AUTOMATIC_TRANSLATIONS_UPDATE config variable should be in place
  at /etc/default/koha-common
- Set AUTOMATIC_TRANSLATIONS_UPDATE to 'yes'
- Re-install the package to trigger the post-install script
- Verify that translations get updated.

Regards
To+

Sponsored-by: Universidad Nacional de Cordoba