[UX] Move the upgrade flow into GitLab.com

Proposal

This is the UX issue for customers-gitlab-com#1993 (closed)

A couple of notes:

1. We will handle the EoA for Bronze use case in a separate issue, this issue is just for a typical upgrade.
2. The scope of this issue is just the upgrade checkout and success message. The billing page (soon the subscription & purchases page) where you click the button to upgrade will be tackled in another issue. The page will be changing, but the information will remain largely the same (ie, you will still see a card that details the features of the Ultimate plan and click a button that says Upgrade).
3. These designs assume that we want to prioritize adding the ability to add seats during upgrade (which is still an outstanding question, see more details in this comment).

Design

• See design section for overall flow and discussion
• See Figma for implementation specs
  • Sign up for a free account with your gitlab email and you can view specs in Figma