Add user event tracking for Dashboard Designer

Proposal

Track the following events using GitLab's internal event tracking:

1. count of users who save a new dashboard (so we know how many users complete the task and can build a funnel later)
2. count of users who save/edit/update an existing dashboard (so we know how often uses need to change dashboards, not as important as the counts above).
3. count of users who visit the dashboard designer (so we can build a funnel later)
4. count of users who visit a custom dashboard (different from count of users who visit any dashboard so we can see if custom dashboards are more valuable to users over time and encourage that behavior)

These are roughly stack ranked in order of importance so we can break out any/all of these into their own issues as needed and deliver them in that order.

Implementation plan

frontend weight 2️⃣

Use internal event tracking to count the following events:

Tracking	Event_ID (suggestion)	Tracking location
User saves dashboard	user_created_custom_dashboard	frontend
User edits dashboard	user_edited_custom_dashboard	frontend
User views dashboard designer	user_viewed_custom_dashboard_designer	frontend
User views custom dashboard	user_viewed_custom_dashboard	frontend

Note: Tracking needs to happen in the frontend because analytics dashboard is a single-page-application and the dashboards are saved and modified using the generic commit API.