Move subscriptions_controller to gitlab_subscriptions

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.