Allow hiding the option to process cash payments for pizza/sales

[DeD1rk]

What?

CAD would like it if we can configure for a food event (and preferably also sales shift) that there's no button to process cash payments. So we could add bool fields (similar to the existing 'allow thalia pay' fields) that determine whether the 'process cash payment' button is shown in the backend and on thadmin.

Why?

CAD wants to disallow cash altogether often (for easier bookkeeping) and want to prevent confusing CAD members by still having it be possible, and to prevent accidentally making cash payments.

Request from Wouter van Ouwerkerk