Refactor(Purchase): consolidate step tracking

What does this MR do and why?

This MR consolidates tracking within the purchase experience to make it easier to follow whilst still tracking important steps

Changelog: changed EE: true

Related: https://gitlab.com/gitlab-org/gitlab/-/issues/457567+.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

New subscription purchase (no visual change)	Addons purchase (no visual change)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Setup tracking debugger locally. Note: Still working to get this completely working for myself!
   1. Setup snowplow - you need to manually set the following in your gdk.yml:

   snowplow_micro:
     enabled: true
     port: 9091

   1. install the snowplow debugger extension.
   2. Navigate to http://localhost:9091/micro/ui to see the events
2. Run gdk rails c and then Feature.enable(:usage_quotas_for_all_editions) to ensure this feature flag is enabled (I believe this is necessary now to see the /buy_storage page: !148694 (merged)
3. Ensure your group does not have a subscription tied to it (cancel it in https://test.zuora.com/ if necessary)
4. Visit the purchase pages, type in your information, and use a card from the stripe test cards:
5. http://localhost:3000/-/subscriptions/new?plan_id=2c92c0f876e0f4cc0176e176a08f1b70&test=capabilities
6. http://localhost:3000/-/subscriptions/buy_storage?selected_group=33
7. Click the Review your order button.
8. See the success/error message and event tracked in http://localhost:9091/micro/ui

Related to #455267 (closed)