Rename base product to parent product in BasketForm

This does not change any functionality, just updates to the adopted
naming scheme.
Add product structure to sandbox fixtures

The T-Shirts were shown as not available; this commit fixes it.
Remove product overview section in product editing dashboard

It is a nice thought, but an uncommon pattern. It also does not add any
information over the 'View on site' public facing site. So let's KISS.
Reorder and rename product dashboard sections

This rearrangement was suggested by Tangent's UX team to try to model a
flow that's closer to how the mental process of adding a product goes.
Hide stock-related fields from display if not tracking stock

All three fields are irrelevant when not tracking stock, so hiding them
avoids confusion. Unfortunately, we need to inspect the product class to
decide whether to hide them or not, as we don't have a form to inspect
when rendering the table header.
Add semi-helpful pointer on how to create child products

It's currently a pain to create child products and will be addressed as
part of #675. We can nonetheless at least try to be helpful and explain
how to get things working till then.
Hide attributes section if product class doesn't define any

If the product class doesn't contain any attributes, the attributes
section in the product dashboard is empty and only serves to confuse.