Show pupil balances in POS

[will-emmerson]

The balance needs to be visible in the POS - most likely this will be at the point where the till operator comes to validate the sale.
It's probably not a good idea to show pupil balances on the customer selection screen, because it would allow one pupil to view another pupils balance.
The most sensible place is to put it in the 'Cashless' button on checkout screen.
This relies on #2 being done.