## 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/test/test_insight.py
"math": None,
                            "name": "$rageclick",
                            "type": "events",
                            "order": 2,
                            "properties": [],
                            "math_property": None,
                        },
                    ],
                    "display": "FunnelViz",
                    "interval": "day",
                    "date_from": "-30d",
                    "actions": [],
                    "new_entity": [],
                    "layout": "horizontal",
                },
                "name": "My Funnel One",
                "dashboard": dashboard.pk,
            },
        )
        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
        objects = Insight.objects.all()
        self.assertEqual(len(objects), 1)
        self.assertEqual(objects[0].filters["events"][1]["id"], "$rageclick")
        self.assertEqual(objects[0].filters["display"], "FunnelViz")
        self.assertEqual(objects[0].filters["interval"], "day")
        self.assertEqual(objects[0].filters["date_from"], "-30d")
        self.assertEqual(objects[0].filters["layout"], "horizontal")
        self.assertEqual(len(objects[0].short_id), 8)
    @patch("posthog.api.insight.update_insight_cache", wraps=update_insight_cache)
    def test_insight_refreshing(self, spy_update_insight_cache):