Create group analytics dashboards page behind feature flag

What does this MR do and why?

• Adds new group_analytics_dashboards feature flag.
• Renders a new blank page at groups/GROUPNAME/-/analytics/dashboards.
• Without the feature flag enabled, it will continue to redirect to groups/GROUPNAME/-/analytics/dashboards/value_streams_dashboard.

How to set up and validate locally

1. Feature.enable(:group_analytics_dashboards)
2. Visit groups/GROUPNAME/-/analytics/dashboards and see the skeleton app
3. Feature.disable(:group_analytics_dashboards)
4. Visit groups/GROUPNAME/-/analytics/dashboards and validate that it redirects to the value streams dashboard instead

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 #390542 (closed)