Move New GitLab.com Subscription flows to GitLab.com UI

Problem

When a new GitLab.com subscriber enters the purchase flow from our pricing page or from the Web Store, if they do not already have a group created, they are blocked from continuing their purchase.

Proposal

Move the GitLab.com New Subscription purchase flow to follow the same flow introduced if the user attempts to purchase via a GitLab.com CTA.

It's managed under move_gitlab_purchases_to_gitlab feature flag.

Success Criteria

• The user should remain in GitLab.com from a UI perspective
• The user should not be prompted to log into the Web Store
• If a group is not yet created, one should be created for them