Add basic order edit functionality to backend

Todo:
- [x] extend logging system for products,
- [x] add logging system for orders and related stuff,
- [x] allow editing adresses (billing / shipping),
- [x] allow edit quantity & price of order items,
- [x] allow change order state,
- [x] allow removal of order item,
- [x] allow adding order items,
- [x] allow changing shipping method,
- [x] allow changing currency(?),
- [x] mark shipped orders as immutable
Hardcoded model class. This is big issue and will break if `ProductVariant` is overridden, which was my case.