PBD: Enforce one stock record matching users partners
Pass user into StockRecordForm

This would be necessary if one wishes to limit the partner selection
available in the product dashboard. As that might be a common
requirement, we just pass it along.
Pre-select partner if appropriate

If the user is not a staff user and has only one partner associated, we
pre-select that.
Extract setting initial data into own function

That makes it possible to override.