This restriction is currently enforced in RT::SharedSetting->_GetObject,
and means that granting SeeGroupDashboard globally (on RT::System) means
"you can see group dashboards for all groups you are in", as opposed to
the previous implementation which caused it to mean "you can see all
group dashboards."

We may move eventually to SeeGroupDashboard globally having the latter
meaning, but that would be a semantic change, and would require other UI
changes.