VSD - Add value stream dashboard enabled/disabled toggle to namespace settings

Various value stream dashboard features will require background data aggregation. To avoid unnecessary data collection and wasteful processing, we've decided to put the feature behind a toggle/checkbox.

Task: Add a new setting to the Group settings page

1. Add the value_stream_dashboard_enabled (default false) boolean column to namespace_settings.
2. Add a DB check constraint to ensure: the flag can be only true when it's a top-level Group (type = 'Group' and parent_id IS NULL).
3. Add the setting to the UI (Group edit page), under Customer relations add a new entry with a checkbox: Value Stream Dashboard (only if the group is a top-level group).
4. Add a feature flag (disabled) to hide the checkbox.
5. Ensure that the VSD routes / menu-items respecting the setting (menu item to be implemented here: #393133 (closed)).

Related items:

• High-level implementation plan: #394705 (comment 1317250071)
• Discussion about the toggle: #394705 (comment 1322313958)