## 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
insight = cast(Insight, insight_serializer.instance)
                    
                    self._attempt_set_tags(new_tags, insight, force_create=True)
                    DashboardTile.objects.create(
                        dashboard=dashboard, insight=insight, layouts=existing_tile.layouts, color=existing_tile.color,
                    )
            except Dashboard.DoesNotExist:
                raise serializers.ValidationError({"use_dashboard": "Invalid value provided"})
        elif request.data.get("items"):
            for item in request.data["items"]:
                insight = Insight.objects.create(
                    **{
                        key: value
                        for key, value in item.items()
                        if key not in ("id", "deleted", "dashboard", "team", "layout", "color")
                    },
                    team=team,
                )
                DashboardTile.objects.create(
                    dashboard=dashboard, insight=insight, layouts=item.get("layouts"), color=item.get("color"),
                )
        
        self._attempt_set_tags(tags, dashboard)
        report_user_action(
            request.user,
            "dashboard created",
            {
                **dashboard.get_analytics_metadata(),
                "from_template": bool(use_template),
                "template_key": use_template,