Use gitlab-ui settings instead for billing page

What does this MR do and why?

• added to gitlab-ui and we can now change and update the colors just a bit to use purple instead of indigo
• uses of these changes are behind a feature flag/experiment(disabled still) - so no changelog

Related to gitlab-ui#1792 (closed)

Screenshots or screen recordings

before	after

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:promote_premium_billing_page)

2. Turn on billing features in the app(via rails console)

   ApplicationSetting.first.update(check_namespace_plan: true)

3. Setup CustomersDot locally
4. Configure to act as SaaS and restart gdk.
5. Create a new group.
6. Visit that group's overview page such as http://localhost:3000/flightjs
7. Find the billing link under settings.
8. Click the billing link.
9. Observe the new billing page.

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.