Improve UX for addons purchase

What does this MR do and why?

Hide labels (formula and text in the summary section) and show a hint for users if invalid quantity is provided during CI Minutes or Storage purchase

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/337632

Screenshots or screen recordings

Before

After

Screen_Recording_2021-10-04_at_14.20.08

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

• Run Feature.enable(:new_route_ci_minutes_purchase) to enable the feature flag for CI Minutes flow

1. Navigate to http://localhost:3000/-/subscriptions/buy_storage?selected_group=<id_of_an_existing_group>
2. Navigate to http://localhost:3000/-/subscriptions/buy_minutes?selected_group=<id_of_an_existing_group>

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.