Bug 19475: Don't copy holiday if it already exists in target calendar

Bug 19475: Don't copy holiday if it already exists in target calendar

Calendars copy tool created duplicate values to database.
Holidays in target calendar weren't checked before
inserting new holidays. This patch fixes this.

To test:
1. Add holidays for branch A
2. Copy branch A calendar to branch B
3. Repeat copy to branch B
=> Check database, branch B has now duplicate holidays
4. Delete holidays from branches A and B
5. Apply patch
6. Repeat steps 1-3
=> Check database, no duplicates

Sponsored-by: Koha-Suomi Oy
Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>