Add management command to renew schedule

Currently, when a schedule is created or updated with "until": null, timeslots until the end of the year are created if the AUTO_SET_UNTIL_DATE_TO_END_OF_YEAR = True, otherwise AUTO_SET_UNTIL_DATE_TO_DAYS_IN_FUTURE days are added to the first_date of the schedule and timeslots are generated for this time span.

At Radio Helsinki we'll need a management command to renew all the schedules that don't have an end_date by adding new timeslots for a to be defined period of time.