## 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
if self.context["view"].action == "list":
            return None
        
        self.context.update({"dashboard": dashboard})
        tiles = (
            DashboardTile.objects.filter(dashboard=dashboard)
            .select_related("insight__created_by", "insight__last_modified_by", "insight__team__organization")
            .prefetch_related("insight__dashboards__team__organization")
            .order_by("insight__order")
        )
        insights = []
        for tile in tiles:
            if tile.insight:
                insight = tile.insight
                layouts = tile.layouts
                
                if isinstance(layouts, str):
                    layouts = json.loads(layouts)
                color = tile.color
                
                if not insight.filters.get("insight"):
                    insight.filters["insight"] = INSIGHT_TRENDS
                    insight.save(update_fields=["filters"])
                self.context.update({"filters_hash": tile.filters_hash})
                insight_data = InsightSerializer(insight, many=False, context=self.context).data
                insight_data["layouts"] = layouts
                insight_data["color"] = color
                insights.append(insight_data)
        return insights
    def get_effective_privilege_level(self, dashboard: Dashboard) -> Dashboard.PrivilegeLevel: