Bring code up to snuff

The four tools have been run and the found issues have been fixed.

In addition, some deprecated uses of the Unzer SDK around the handling of the basket have been fixed.
One side effect of this is that there is now a stricter validation on the Unzer side.
Basket items with a value of zero are no longer allowed and will now be skipped
when we create the basket.

Automatically closed - issue fixed for 2 weeks with no activity.