Update to Mollie v2 API: Part 2 electric boogaloo

[leuke-naam]

Fixes issues with #1133

[SilasPeters]

Lgtm

[SilasPeters]

@leuke-naam and @TobiasDeBruijn I pulled a big brain and tested this branch on staging without merging, by simply checking out this branch and restarting koala 🧠 . Everything seems to work! Two things though:

1. If the payment is left as 'open', the user is not notified, but I believe @leuke-naam showed me once that a blue banner displaying 'your payment is being processed' should pop up. If that's true, that did not work.
2. The payments are in test mode, but I assume this is toggled off when a different MOLLIE_TOKEN is used in .env in production.

[leuke-naam]

@leuke-naam and @TobiasDeBruijn I pulled a big brain and tested this branch on staging without merging, by simply checking out this branch and restarting koala 🧠 . Everything seems to work! Two things though:

1. If the payment is left as 'open', the user is not notified, but I believe @leuke-naam showed me once that a blue banner displaying 'your payment is being processed' should pop up. If that's true, that did not work.
2. The payments are in test mode, but I assume this is toggled off when a different MOLLIE_TOKEN is used in .env in production.

1. Payment status open is not applicable for iDeal payments, if in the future we were to accept more payment types we could handle this status
2. Correct

[SilasPeters]

Then everything works! Somehow GitHub complains some requirements for merging are not met, so I will admin abuse