Upgrade to EveryPay APIv4

[vohmar]

in november 2021 the support for v3 of the everypay API will end so all integrations should be upgraded to v4.

doc:
https://support.every-pay.com/downloads/everypay_apiv4_integration_documentation
https://support.every-pay.com/ms/integration/

v4 enables using linkpay and open banking features so we can drop using swedbank's and seb's banklinks and direct all payments to our primary bank account. WE can also add linkpay feature to all emails and pdf invoices.