New payment system

[ShadowDog007]

Key changes:

• Payments are saved in a single row at the time of the payment
• Payment is returned from mutation so we can see the actual amount paid
• Payments use decimal for amount, instead of doubles (why fraser why)

Migration: Currently detect pending changes, if the payment migration was run, create payments based off amounts stored in legacy format.

[ShadowDog007]

Would like feedback around behavior of payment when clicking 'Pay All'.

1. 'Pay All' takes existing behavior where it pays what the backend thinks is owed (not what was shown / would have been paid) (We could then show the actual amount 'paid' which could differ, and cause problems?)
2. 'Pay All' sends the amount shown on the page as a payment, and is blindly accepted (Amount owed is updated)

I'm leaning towards the 2nd solution, which will require a little more work and will break the frontend until it's updated. (Current changes might break too sooo?)