Figure out if `schedule_future_zimfile_generations` should belong in queues.py

[audiodude]

It doesn't quite fit the "shape" of the other functions in that file, it seems to be doing too much. Obviously it has to access the queue/scheduler, which belongs in that file. But it's also creating and saving a ZimSchedule, which feels like it should be done in a wrapper that calls this function in queues, instead of inside it itself.

[nicolapace]

I agree, I would do it in zim_schedules.py, calling a small function in queues.py