Restyle signup welcome page

This is a redesign of the signup welcome page, part of the paid signup flow (gitlab-org/growth/product#87 (closed))

Current design:

New design desktop: https://gitlab-org.gitlab.io/gitlab-design/hosted/kcomoli/growth%2387-gitlab-com-purchase-experience--specs-preview/#artboard17

New design mobile: https://gitlab-org.gitlab.io/gitlab-design/hosted/kcomoli/growth%2387-gitlab-com-purchase-experience--specs-preview/#artboard0

Implementation in this MR:

Desktop	Mobile

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

To test this feature in development setup:

• Sign in and visit http://localhost:3000/users/sign_up/welcome