Add task to clean database using cron schedule

[simonhammes]

No description provided.

[christophdb]

Most users forget to create a cronjob to clean their database regularly. To avoid that database is blotted with operation log after some weeks of months, SeaTable should clean the database itself.

This adds a dtable-event job, running every night at 0:30 am. It uses the retention days, that are currently part of the clean_db script.

Still the retention days could be customized in dtable_events.conf with

[CLEAN DB]
keep_dtable_snapshot_days = 365
keep_activities_days = 30
keep_operation_log_days = 14
keep_delete_operation_log_days = 30
keep_notifications_usernotification_days = 30
keep_dtable_notifications_days = 30
keep_session_log_days = 30