Support binding arbitrary data to a future payment

[Klakurka]

Let's do this our way first, then eventually support BIP 70 when we're able to.

Client task: PayButton/paybutton#66

This data would be viewable in admin for the payment it's attached to (likely using the payment id which we can start providing from the server instead of having the client generate it) but also perhaps we can add a new dialog somewhere for incomplete payments to allow admins to view data that was submitted but where the payment didn't happen.

Ideally, this feature would do everything needed to replace the Coin Dance supporter form although we will probably need to do this in phases. For v1 we just need to allow for some simple data inputs.