This will let us branch by abstraction. All existing calls to
`#outstanding_balance` will go through `OrderBalance` hence, will
check the feature toggle.
Note that by default, `OrderBalance` will end up calling
`#old_outstanding_balance`. As the name states, that's exactly what
`#outstanding_balance` was so far. This means no consumers will see any
change in behavior. We just added on item in the call stack (sort of).
- Partial is used by both the invoice pdf and the order confirmation email
- separate scss file for new payment list table
- extracted outstanding balance logic (also changed in payments view.. admin/orders/RXXX/payments)
- translations in shared.payments_list and lazy loaded
