Stockrecord handling in permission-based dashboard 

@mbertheau highlighted some issues with the ProductCreateUpdateView for stock record handling. The following issues need to be addressed. Some of that work was already done as part of the PBD and was forgotten when merging multiple stock record support into the PBD branch.

- [x] Enforce at least one stockrecord matches. Apart from that, don't limit partner queries. setting non-matching partners is allowed.
- [x] Creating products: Pre-select matching partner for first stock record if there's only one partner matching. Example in https://github.com/mbertheau/django-oscar/commit/2f11953bebca7112df03a7a267ea98c5ebc222fb, but probably best done on formset creation
- [x] Ensure customising partner queryset (e.g. limiting to matching partners) is easily done