Include coupon code as query param in stepper onboarding checkout

[briowill]

Proposed Changes

Coupons applied in Stepper Onboarding flow are not applied to final price in Checkout.
This PR applies the coupon code to the final price in Checkout

Testing Instructions

1. Checkout branch
2. Launch Stepper onboard flow and include a coupon code calypso.localhost:3000/setup/onboarding/?coupon={code}
3. Complete onboard flow
4. Discount seen at pricing page should be reflected in checkout total

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-880a28bb6b6af1eebd1bcefea9f3c268cf91a63d

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-880a28bb6b6af1eebd1bcefea9f3c268cf91a63d&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:commit-880a28bb6b6af1eebd1bcefea9f3c268cf91a63d&env=a8c-for-agencies

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~20 bytes added 📈 [gzipped])

name             parsed_size           gzip_size
onboarding-flow        +68 B  (+0.2%)      +20 B  (+0.2%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[briowill]

Closing as not needed now p1724991591885219-slack-CV1BNGVCH