Update active status on product quantity change

[Thizizmyname]

If a product is inactive and it's quantity amount is changed it should be automatically set to active again so it's quantity is confirmed in the next stock-take.

This is an expansion for #75.

There are three moments a quantity is changed:

• When a product is purchased
• When a delivery is finalized
• When an admin manually changes the amount of the product

A purchased object gets changed on "finalize_purchase" in
foobar_api/src/foobar/api.py

A delivery changes a product on "populate_delivery" in
foobar_api/src/shop/api.py