Add group scoped feature flag for wi boards

What does this MR do and why?

When we gradually roll out the new planning view work item boards feature, we'll need to do so on a root level group basis - as other percentage based rollouts will result in unusual and inconsistent behaviour.

This MR adds a new 'group' feature flag that mirrors the existing user-scoped planning_view_boards feature flag, but for root groups only.

If either of the user or group scoped feature flags are enabled in the request context, the feature will be enabled.

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