[FE] Breakdown: Move the GitLab.com upgrade purchase flow to GitLab.com

Proposal

Move the GitLab.com upgrade purchase flow to GitLab.com CTA.

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
  • The user should not be prompted to enter a namespace for the purchase

Design

See design issue

Edited by Tatyana Golubeva