The donation bounded context contains a class UniqueTransferCodeGenerator that that is a wrapper around payment reference codes (formerly known as transfer codes) that ensures they're unique, by looking at the database and generating new ones until a unique one is found. This functionality must now be in the payment domain

Acceptance criteria:

• Functionality is in the payment domain and passes tests

Implementation notes:

You can no longer use an entity repository. Instead, use a raw SQL and a UNION to look for transfer codes both in sofort and bank transfer tables