task chunk series

[wichmannpas]

similar to #1, but for task chunks.
Allow to schedule a task chunk series.

Add a TaskChunkSeries model (or similar). It contains a rule (e.g., every 7 days, starting on 2018/02/03, forever; or every first saturday of the month).
Expand (create corresponding task chunks) for example 5 weeks into the future or for the next N occurences; expand further by cron or similar as the time advances.