Discount After Gratuity

Linga already has an option to apply the discount before or after tax. 

We also need an option to apply the discount Before and After Gratuity

Plenty of restaurants are running promos and events where half the food is free, but they don't want the servers to miss out on the Gratuity since they don't work any less on the events and banquets.

This will also, indirectly, help with the deposit handling.