Add e2e tests for purchasing CI minutes

What does this MR do and why?

Part of customers-gitlab-com#3411 (closed)

• Migrate CI minutes purchase flow to GitLab repo
• Add related ChemLab pages

How to set up and validate locally

run

bundle exec bin/qa Test::Instance::All https://staging.gitlab.com -- qa/specs/features/ee/browser_ui/fulfillment/purchase/purchase_ci_spec.rb

Numbered steps to set up and validate the change are strongly suggested.

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.