BUGFIX: admins were unable to change the GL.com plan for a namespace

What does this MR do?

It restores the ability to change a GL.com plan for a Group or User. It was broken because recently we've changed the way we store information about a Subscription for a Group or User, now we use the gitlab_subscriptions table instead of the plan_id column from namespaces.

What are the relevant issue numbers?

Closes #8901

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer