Show coupon code input for small breakpoints

What does this MR do and why?

Adds coupon code field for purchase flow on small breakpoints. This MR is about adding the field only, it will be wired up in a follow-up MR.

Screenshots or screen recordings

Scenario	Before	After
Small breakpoint confirm order
Small breakpoint when editing subscription details
Small breakpoint when editing billing address
Small breakpoint when editing payment method
Large breakpoint (no change)
CI minutes purchase initial collapsed state (no change)	-
CI minutes purchase expanded state (no change)

How to set up and validate locally

https://gitlab.com/gitlab-org/gitlab/-/issues/385366#how-to-set-up-and-validate-the-changes-locally

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #385366