IACO-535 integrate with sample backend

Sadly, this review request contains a lot of noise, mainly due to some left over files I discarded and a new pod I added to the workspace.

Do give the important bits a glance, and don't forget to check out the [sample backend](https://github.com/klarna/sample-app-backend)'s code as well. The sample backend will also need to have some documentation, which I will get cracking on.
1. I would move some of the logic to the server, basically just passing the reference, and the server will know everything by the reference.
2. If not, calculate the total and the taxTotal from the proof performPurchaseOf value