Disallow selecting child products as parent

Prior to this change, it was possible to select child products in the
product dashboard. A new function get_parent_products_queryset was
introduced, analogues to the existing get_related_products_queryset. It
is not based on the BrowsableManager as I can envision scenarios where
that is extended to respect status fields or such like, which we should
ignore when selecting a parent field. Otherwise it wouldn't be possible
to e.g. add a few hidden products with child products.

Two issues in tests with the G library were fixed as well, were
unintentionally child products were created. If the product field is
left unspecified, it seems to pick a product instead of leaving it as
None.