Product Analytics dashboard basic structure

What does this MR do and why?

Related to #370556 (closed)

Adds an EE controller for Product Analytics.

This controller renders the Project / Product Analytics / Dashboards page. Temporarily extending the former route. This is behind the product_analytics_internal_preview feature flag and the product_analytics licensed feature.

It also renders a basic Vue app with an empty element on "Project / Product Analytics / Dashboards". To be extended on Product Analytics dashboard widgets structure (!98613 - merged).

Screenshots or screen recordings

Screenshot

Note that it currently activates the Monitor / Product Analytics menu item. This will be addressed at Left sidebar organization for Product Analytics (#375736 - closed).

How to set up and validate locally

1. Make sure you are on GitLab Ultimate.
2. Enable the product_analytics_internal_preview feature flag

   Feature.enable(:product_analytics_internal_preview)

3. Visit any PA project pages such as http://127.0.0.1:3000/groups/flightjs/Flight/-/product_analytics/dashboards

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.