Handle data migration to multi chamber
Created by: njoyard
One thing I think we overlooked when coding+merging multi chamber: data migration. We should ensure that:
- Memopol behaves correctly when using multi-chamber code with EP-only data
- Chamber, group & mandate creation behave correctly on first update (this is more an issue for django-representatives)
Maybe creating chamber+mandates in the django migration would be a good solution ?