Core/Events: allow to override holiday durations and correct Lunar Festival duration down to 2 weeks

* the new field must be set for every new holiday date entry to take effect if you want to override the duration of the given

sql/updates/world/custom/custom_2020_01_11_05_world.sql

@@ -0,0 +1,2 @@
+ALTER TABLE `holiday_dates` ADD COLUMN `holiday_duration` INT(10) UNSIGNED NOT NULL AFTER `date_value`;
+UPDATE `holiday_dates` SET `holiday_duration`= 360 WHERE `id`= 327;