Implement show repetitions defined as schedules

Parent Task: Repeating Schedules

For every schedule there may be a repeating schedule. In theory you could also just create a distinct schedule and tell it to be a repetition schedule.

A repetition schedule is signified by setting the is_repetition flag to true. It means that for each timeslot in the repetition schedule the content should be take from the last timeslot of the show before the repetition timeslot, which has the is_repetition set to false.

For some uses cases there are the helper parameters add_days_no (int) and add_business_days_only (bool). This is explained in comment of the models.py in steering

Use cases we would like to have:

• Click on an existing timeslot and choose to generate a repetition timeslot
  • Choose "On the following day" (with optionally different time)
  • Choose "On the following business day" (with optionally different time)
  • Choose "... days later" with a checkbox for choosing only to use business days
  • Choose your own schedule parameters for the repetition

When creating a repetition, it should automatically copy all exising data to the new timeslot(s).