New PaymentMethod Model

[jsayles]

We need to capture multiple types of payments and indicate on the Payment which method was used.

Example Payment Methods: Visa, Master Card, American Express, Cash, Check, Interac, etc.

This needs to be captured when payments are recorded.

[jsayles]

Since PaymentMethods won't change frequently, we can rely on the admin interface to enter new ones.

[tnightingale]

Initial model: https://github.com/nadineproject/nadine/blob/payments/nadine/models/billing.py#L714-L722