Move subscriptions controller to gitlab_subscriptions

What does this MR do and why?

We want to consolidate our namespace/bounded context strategy around gitlab_subscriptions.

• Move ee/app/controllers/subscriptions_controller.rb to ee/app/controllers/gitlab_subscriptions/subscriptions_controller.rb and use the GitlabSubscriptions namespace.
• Move the routing definition from ee/config/routes/subscription.rb to ee/config/routes/gitlab_subscriptions.rb.
  • Do not change the URL. So we can use resource module: :gitlab_subscriptions here to probably handle that.

References

Please include cross links to any resources that are relevant to this MR This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Routes before	Routes after

Related to #461883 (closed)