Show banner after storage purchase

What does this MR do and why?

After successful storage purchase redirect to storage tab in Usage quotas and show an alert

Related issue: #327895 (closed)

Screenshots or screen recordings

Purchase	Notification

How to set up and validate locally

1. Run CustomersDot locally
2. On your local GitLab instance enable a feature toggle
   • Run rails c
   • Run Feature.enable(:new_route_storage_purchase) to enable the feature flag for Storage flow
3. Navigate to http://localhost:3000/-/subscriptions/buy_storage?selected_group=<id_of_an_existing_group>
4. Complete purchase
5. Check that you are redirected and the notification is shown

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.