Refactor / align payment (upgrade) integration flows

[ChristiaanScheermeijer]

Description

This proposal PR is my attempt to align the subscription switch logic discrepancy between the JWP and Cleeng integration. These two features (doing the same) were implemented differently.

Motivated by #562 and #555 I'm hoping we can simplify flows and eliminate differences between providers. To disable features, we should be using the integration feature flags to keep the code intact for other providers (as long we don't have alternatives).