Thalia Pay: Add a direct debit payment method to the payment model

Ghost User
2019-11-06T19:31:20+01:00

Subtask of #632

### One-sentence description

Add a direct debit payment method to the payment model

### Motivation

Because we want to introduce Thalia Pay (#632)

### Desired functionality

The Payment model should have an option to mark it as paid using direct debit.

### Suggested implementation

Just like the other payment methods.