Fix SSO subscription registration dropoff

What does this MR do and why?

Fixes the SSO subscription registration dropoff(clicking social links on sign in page after redirect from /-/subscriptions/new if not logged in).

See larger explanation in Fix SSO subscription registration onboarding dr... (#413995 - closed)

Screenshots or screen recordings

screenshots taken from capturing screenshots from ee/spec/features/registrations/saas/subscription_flow_company_paid_plan_spec.rb

page	Before	After
welcome page
company form		N/A - subscriptions gather info differently
checkout	N/A(broken out of subscriptions as this point)

How to set up and validate locally

Hard to validate locally unless customersDot is fully setup to take subscriptions, but our changes are before checkout, so tests we have will cover - full test path can be seen in ee/spec/features/registrations/saas/subscription_flow_company_paid_plan_spec.rb

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 #413995 (closed)