Forward promo_code param from group selector

What does this MR do and why?

Allows us to set promo_code param in the subscription new group flow. This param is forwarded to CustomersDot and allows us to auto-apply it.

References

Issue: https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/13901

Screenshots or screen recordings

How to set up and validate locally

1. Visit http://localhost:3000/-/subscriptions/groups/new?plan_id=2c92a00d76f0d5060176f2fb0a5029ff&promo_code=SAAS_DISCOUNT
2. Test selecting an existing group and creating a new group
3. You should be redirected to CustomersDot with the promo_code param forwarded. i.e. http://localhost:5000/subscriptions/new?plan_id=2c92a00d76f0d5060176f2fb0a5029ff&gl_namespace_id=160&promo_code=SAAS_DISCOUNT

MR acceptance checklist

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