Add MembershipPayment model

We need a model for a single membership payment, that will be created every time fees have to be paid (e.g. every year or so).

The model needs:

• A foreign key to Membership
• A field holding the date when the payment is due
• The amount (money field)

It needs to implement the Tezor for_object API (by exposing the required methods/properties), so we can make Tezor payments for membership payments.

No frontend is required right now.