Show expiration date for non-prorated purchases

What does this MR do and why?

Show subscription dates for all Storage purchases (not just prorated ones)

Related to #327896 (comment 808721969)

Screenshots or screen recordings

Dates for prorated storage ()	Dates for not-prorated storage

How to set up and validate locally

1. Run CustomersDot and GitLab locally
2. On your local GitLab instance enable a feature toggle
3. Run rails c
4. Run Feature.enable(:new_route_storage_purchase) to enable the feature flag for Storage flow
5. Make sure that you have correct settings Gitlab::CurrentSettings.update(automatic_purchased_storage_allocation: true)
6. Find an existing group with a subscription (Premium or Ultimate)
7. Open this group page and navigate to Settings -> Usage Quotas -> Storage click “Purchase more storage” OR use a fast forward link http://localhost:3000/-/subscriptions/buy_storage?selected_group=<id_of_an_existing_group>
8. Check that the dates are shown and the Storage subscription ends at the same date as the main subscription AND
9. Create a new group
10. Open this group page and navigate to Settings -> Usage Quotas -> Storage click “Purchase more storage” OR use a fast forward link http://localhost:3000/-/subscriptions/buy_storage?selected_group=<id_of_an_existing_group>
11. Check that the dates are shown and it's a full year starting from today

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.