[Feature request] Make unit price adjustments optional for non admins

[CabtonWithaK]

Context as an admin I'm using LNBits to manage different TPOS instances. Point of sales are operated by coworkers.

AS IS: As an admin I can't prevent the unit price of items sold to be changed by a POS operator.
TO BE: As an admin I can set the option to block an operator from adjusting the unit price.

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** AS IS **

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** TO BE **

option added to TPOS config