Multi-tenant billing

[turt2live]

If enabled, the server should be able to set up pay walls to use the bridge when in MT mode. This will mean that users are required to pay for the services used, potentially with a service fee on top to keep the bridge running.

The payment processor should be abstracted out. The payment model would also ideally be abstracted.

The default payment model could be:

• Users pay for the phone number at cost
• Users pay the message rate +X% for incoming/outgoing messages (the X% goes to operation costs)
• Credit-based model (fill up the bucket before using it)
• At $0 credit left, suspend service for X days before canceling the number.