Add seats addition tracking to Gitlab.com flows

What does this MR do and why?

Add tracking to Add seats button clicks on Billing page and on Usage quotas page.

Event details for billing page:

  event_category = 'Webstore' 
  event_action = 'click_button' 
  event_label = 'add_seats_saas'
  event_property = 'billing_page'

Event details for usage quotas page:

  event_category = 'Webstore' 
  event_action = 'click_button' 
  event_label = 'add_seats_saas'
  event_property = 'usage_quotas_page'

Screenshots or screen recordings

Screenshot	Tracking

How to set up and validate locally

1. Make sure that you have CustomersDot instance setup and running
2. Install Snowplow Debugger
3. Find or create a new SaaS subscription for any group
4. Open Billing page http://localhost:3000/groups/your-group/-/billings and click the Add seats button in the table below
5. Ope Usage page http://localhost:3000/groups/nov22-1-user-premium/-/usage_quotas and click Add seats button
6. For both buttons you should see the Structured Event in the Snowplow debugger

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.

Related to #408956 (closed)