Fix mariadb timestamp columns

MariaDB automatically inserts the current time, even when saying `SET
datecol = null`.

modules/store/src/main/resources/db/migration/mariadb/V1.6.1__fix_timestamp_columns.sql

@@ -0,0 +1,14 @@
+ALTER TABLE `item`
+MODIFY `itemdate` TIMESTAMP NULL;
+
+ALTER TABLE `item`
+MODIFY `duedate` TIMESTAMP NULL;
+
+ALTER TABLE `user_`
+MODIFY `lastlogin` TIMESTAMP NULL;
+
+ALTER TABLE `job`
+MODIFY `started` TIMESTAMP NULL;
+
+ALTER TABLE `job`
+MODIFY `finished` TIMESTAMP NULL;