Add user event tracking for Dashboard Designer

What does this MR do and why?

Adds user event tracking for the analytics dashboard designer for Add user event tracking for Dashboard Designer (#420827 - closed).

The following internally tracked events are added:

Description	Event name	Tracking location
User saves dashboard	user_created_custom_dashboard	frontend
User edits dashboard	user_edited_custom_dashboard	frontend
User views dashboard designer	user_viewed_dashboard_designer	frontend
User views custom dashboard	user_viewed_custom_dashboard	frontend

Note that analytics dashboard is an experimental feature hidden behind default off feature flags.

Screenshots or screen recordings

No visual changes.

Before	After

How to set up and validate locally

1. Follow the instructions to debug internal events tracking.
2. Enable custom dashboards on your GDK:
   • echo "Feature.enable(:combined_analytics_dashboards)" | gdk rails c
   • echo "Feature.enable(:combined_analytics_dashboards_editor)" | gdk rails c
3. View the dashboards list at Project > Analytics > Analytics dashboards and click on Configure Dashboard Project.
4. Select any repository to store your custom dashboard files.
5. View the dashboards list at Project > Analytics > Analytics dashboards.
6. Click on New dashboard and create a new dashboard. It should track the user_viewed_custom_dashboard_designer + user_created_custom_dashboard events.
7. Go back to the dashboard list and click on your new dashboard. It should track the user_viewed_custom_dashboard event.
8. Click on the Edit button. It should track the user_viewed_custom_dashboard_designer event.
9. Make some changes and click Save. It should track the user_edited_custom_dashboard event.

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 #420827 (closed)