Fix summary section when purchasing an addon

What does this MR do?

This is the 2nd merge request that adapts the SummaryDetails component for purchasing an addon (CI Minutes or Storage pack). Right now the component has hardcoded values for CI Minutes pack, later we will change them to dynamic string that depend on the picked addon.

Next steps will be adapt all other components in the purchase flow and generalize this workflow for purchasing any addon.

See this epic for more information.

How to test this locally

• Make sure you have the CustomersDot app running locally.

• Make sure your local GDK instance points to your local CustomersDot. You the following lines to the env.runit file in the root folder of your GDK installation:

  export CUSTOMER_PORTAL_URL=http://localhost:5000/

• Enable the following feature flag Feature.enable(:new_route_ci_minutes_purchase)

• Sign in into your local GitLab instance and navigate to -/subscriptions/buy_minutes?namespace_id=NAMESPACE_ID

Related #321648 (closed)

Screenshots (strongly suggested)

Description	Image
Entering quantity
After clicking on Continue to billing
After clicking on Continue to payment
After clicking on Submit

In Action

ci_minutes_purchase-2021-07-08_13.16.58

Does this MR meet the acceptance criteria?

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