Correct the way that PaymentGatewayEntities are saved.

- A new paymentGatewayId is created each time the conference is parsed to a PaymentGatewayEntity. Because of this,
- the PaymentGatewayEntities were not being updated correctly. Instead of updating by UUID, the PGE is now updated by
- PaymentGatewayId (String)

This is now tested as part of the UpdateConference test.
This is fine.