Add `group_analytics_dashboard_editor` feature flag

What does this MR do and why?

Adds a new feature flag that allows the creation of custom analytics dashboards for groups and subgroups.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Feature on	feature off

How to set up and validate locally

• Ensure you are using GitLab Ultimate
• Navigate to any group Analytics dashboard page (ex. http://gdk.test:3000/groups/flightjs/-/analytics/dashboards/)
• Validate that the new dashboard button appears when the feature flag is enabled

Related to #508774 (closed)