ical still refers to peopledoc

[ewjoachim]

Related to #653 : we should somehow migrate peopledoc references into workalendar for ical-related stuff, but this may be a kinda-breaking change. I think this would create duplicate events for users because event IDs are expected to be immutable. But I'm not really sure.

This ticket is to evaluate the impacts, the possible strategies and take decisions before we act. (the associated PR may be a 1-liner).