Add Discount and Promo Code Functionality

[AArchange]

Description: Implement a system for applying discounts and promo codes to invoices.

Tasks:
Create Discount model and migration.
Develop a UI for creating and managing discounts.
Implement functionality to apply discounts to invoices.
Acceptance Criteria:
Users can create and apply discounts or promo codes to invoices.
Discounts are reflected in the invoice total.