Implement promo code functionality

Objective

See the objective outlined in the parent epic [Unify purchase flows] SaaS/Self-Managed Premiu... (&18567).

Purpose

This issue is to implement promo code functionality on the unified version of SaaS and Self-Managed purchase flows with appropriate error handling.

This should also include defaulting promo code param from the URL (when present) in the promo code input box for parity with current functionality.

Designs

Full designs: #10162[Adding_a_promo_code.png]