## Is your feature request related to a problem?

When setting up dashboards I'd like to filter the stats to a certain page/subdomain. 

## Describe the solution you'd like

Allow adding a extra filters for all graphs on a dashboard (e.g. filter by $current_url)

## Describe alternatives you've considered

Updating every graph in dashboard one-by-one and then reverting them back.

## Additional context

Related to https://github.com/PostHog/posthog/issues/780

Datadog has a decent solution for this: https://docs.datadoghq.com/dashboards/template_variables/, which I think we can simplify and use ourselves.

Came up during #2560 

#### *Thank you* for your feature request – we love each and every one!

posthog/api/dashboard.py
queryset = super().get_queryset()
        if not self.action.endswith("update"):
            
            queryset = queryset.filter(deleted=False)
        queryset = queryset.select_related("team__organization", "created_by").prefetch_related(
            Prefetch("insights", queryset=Insight.objects.filter(deleted=False).order_by("order"),),
        )
        return queryset
    def retrieve(self, request: Request, *args: Any, **kwargs: Any) -> response.Response:
        pk = kwargs["pk"]
        queryset = self.get_queryset()
        dashboard = get_object_or_404(queryset, pk=pk)
        dashboard.last_accessed_at = now()
        dashboard.save(update_fields=["last_accessed_at"])
        serializer = DashboardSerializer(dashboard, context={"view": self, "request": request})
        return response.Response(serializer.data)
class LegacyDashboardsViewSet(DashboardsViewSet):