Display billing address in GitLab.com purchase flow

What does this MR do and why?

Related to #433384 (closed) | Designs: Design link

When the key_contacts_management feature flag (currently enabled by default) and key_contacts_management_v2 feature flag are enabled and the customer has a billing account, show the billing&subscription contact information in the Contact information step instead of just the notice.

Also redirects the user to CustomersPortal when clicking the Edit on CustomersPortal button on the contact information step.

⏩ Related: Check mark behavior for contact information step !139603 (merged)

Screenshots or screen recordings

Before	After	After without FF

Edit button behavior for contact info step

	New subscription	Add-on
Step footer edit button	Step footer edit button	addon_subscription

How to set up and validate locally

1. Enable the key_contacts_management_v2 feature flag

   Feature.enable(:key_contacts_management_v2, type: :gitlab_com_derisk)

2. Have CustomersDot set up and connected to GitLab
3. Visit http://localhost:3000/-/subscriptions/new?plan_id=2c92c0f876e0f4cc0176e176a08f1b70 (new subscription)
4. Navigate to the contact information step
5. You should see both a tip at the top and the contact information listed
6. Click the footer edit on customers portal button on the contact information step
7. A new tab for the CustomersPortal Billing Account Settings page should open
8. Visit http://localhost:3000/-/subscriptions/new?plan_id=2c92c0f9699eca030169a2f1f117045f and test the same logic (add-on form)
9. Disable the feature flag

   Feature.disable(:key_contacts_management_v2, type: :gitlab_com_derisk)

10. Verify the form still works as expected

MR acceptance checklist

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