Order summary component for paid signup flow

This provides the order summary component for the paid signup flow

This MR is not functional on it's own, but instead is created in an effort to split up gitlab-org/growth/product#87 (closed) into smaller, more digestable MR's.

Design: https://gitlab-org.gitlab.io/gitlab-design/hosted/kcomoli/growth%2387-gitlab-com-purchase-experience--specs-preview/#artboard15 and https://gitlab-org.gitlab.io/gitlab-design/hosted/kcomoli/growth%2387-gitlab-com-purchase-experience--specs-preview/#artboard5

There is no changelog, since the feature is behind a feature flag.

To see this code in development setup:

• Enable the feature flag in the console: Feature.enable(:paid_signup_flow)
• Add return [{id: 'x', code: 'bronze', price_per_year: 48}] on ee/app/helpers/subscriptions_helper.rb#16 (to avoid having to configure and run the customers app)
• Sign in and visit http://localhost:3000/-/subscriptions/new

Desktop	Mobile

Closes gitlab-org/growth/engineering#5049 (closed)