Zeffy API Integration

[braddoeswebdev]

Zeffy has an API available now that we should integrate with!

https://support.zeffy.com/get-started-with-the-zeffy-api-yourg

[braddoeswebdev]

This is on hold until Zeffy improves their API - there's currently no clean way to pass any kind of custom metadata along through the payment workflow so it doesn't currently work for our primary use case.