holiday_si_sl: updated Slovenian holidays

Summary:
Updated Slovenian holidays and added an extra source, from the official open data portal.

The second day of new year was reinstated as a public holiday.

Reviewers: KDE PIM, winterz

Reviewed By: KDE PIM, winterz

Subscribers: winterz

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D9680