Add account fee to billing statements

The plan is to do this with a simple config field for now, and maybe think about having this be settable via the admin UI if we end up needing that.
I'm tempted to say there has to be a credit amount because those are the only things we can deduct? We should chat a bit about how deducting the account fee will actually work though (I'm assuming we'll deduct it from credits and create a transaction for it, but we should check with @badlard).