Reroute storage to GitLab flow

What does this MR do and why?

We are migrating Storage purchase from CustomersDot to GitLab flow &5789 (closed)

This MR puts condition around purchase storage links to show a GitLab one when new_route_storage_purchase feature flag is on

Related to #327886 (closed)

Screenshots or screen recordings

Case	Screenshot
Storage purchase tab
Storage limit alert

How to set up and validate locally

• Storage purchase tab

1. Make sure that you have storage allocation setting enabled Gitlab::CurrentSettings.update!(automatic_purchased_storage_allocation: true)
2. Enable feature flag Feature.enable(:new_route_storage_purchase)
3. Choose a group and open Settings -> Usage quotas and click Storage tab
4. Link on Purchase more storage should lead to url like http://localhost:3000/-/subscriptions/buy_storage?selected_group=<group_id> and open it in a new tab

• Storage limit alert

1. Enable feature flag Feature.enable(:new_route_storage_purchase)
2. Comment out - return unless display_namespace_storage_limit_alert?(namespace) line in ee/app/views/shared/_namespace_storage_limit_alert.html.haml
3. Open any group page and check the link from an alert

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.