Feature: Toggle usage of coupons via settings

[R4Nilluminatixs]

Make it possible to enable or disable the usage of coupons throughout the shop via the settings.

Closes: feature/toggle-usage-of-coupons

Note: this does keep the coupon elements available for the admin, its pointed towards user interactions and cart calculations.

[R4Nilluminatixs]

^ Added coupon toggle in settings

^ Cart with coupons disabled

[R4Nilluminatixs]

@MrMicky-FR this one was already ready to be reviewed/merged, added some context via the comment.