[W-15226710] Fix promo code application

[bendvc]

Description

This fixes issue #1628

Types of Changes

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Documentation update
• Breaking change (could cause existing functionality to not work as expected)
• Other changes (non-breaking changes that does not fit any of the above)

Changes

• Use async API so that the form handler isn't always a success.

How to Test-Drive This PR

• Add an item to your cart
• View your cart
• In the side bar add a promo code "test"
• You should se an error because there is no promo with that code, but the form is still visible.
• Now try "orderLevel"
• You should see a toast and the form go away, and a successful promo added.
• Remove the promo
• All is good.

Checklists

General

• Changes are covered by test cases
• CHANGELOG.md updated with a short description of changes (not required for documentation updates)

Accessibility Compliance

You must check off all items in one of the follow two lists:

• There are no changes to UI

or...

• Changes were tested with a Screen Reader (iOS VoiceOver or Android Talkback) and had no issues
• Changes comply with WCAG 2.0 guidelines levels A and AA
• Changes to common UI patterns and interactions comply with WAI-ARIA best practices

Localization

• Changes include a UI text update in the Retail React App (which requires translation)