UI for payment method enable/disable

[sde1000]

Add a user interface to the till to allow payment methods to be toggled between "Enabled" and "Totals only".

We have a backup payment method at some pubs — a SumUp card terminal. It must be possible to start using this method when the internet is down, so there has to be some way to turn it on directly on the till without using the web interface.

It's necessary to keep it turned off (rather "Totals only") to prevent members of staff using the payment method by accident (eg. putting a transaction through as SumUp rather than as Cash).