Bug 10050: Update German web installer files for 3.12 - Optional

Bug 10050: Update German web installer files for 3.12 - Optional

This patch updates the German web installer files from the
optional directory.

This includes:
- Adding the bridge/reference.gif to the REF itemtype
- Updating currency rates and setting EUR to active
- Adding branchcode MPL to the sample holidays
- Updating a few strings on the patroncard samples
- Fixing typos and rephrasing

To test:
- Run the web installer with German installed
- Verify all optional SQL files are installed correctly

Note: The German po files in master are buggy, please
use files from translate.koha-community.org for testing.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>