Include the Namespace Id in purchase link for GL.com plans

What does this MR do?

Includes the Namespace Id in purchase link for GL.com plans

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

To avoid confusion and to prevent users selecting the wrong group or personal account in the purchase form used in the subscription portal.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-com/customers-gitlab-com/issues/227