## 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
_create_person(team=self.team, distinct_ids=["1"])
        _create_event(team=self.team, event="user signed up", distinct_id="1")
        _create_event(team=self.team, event="user did things", distinct_id="1")
        response = self.client.post(
            f"/api/projects/{self.team.id}/insights/funnel/",
            {
                "events": [
                    {"id": "user signed up", "type": "events", "order": 0},
                    {"id": "user did things", "type": "events", "order": 1},
                ],
                "funnel_window_days": 14,
            },
        )
        self.assertEqual(response.status_code, status.HTTP_200_OK)
        response_json = response.json()
        
        self.assertEqual(len(response_json["result"]), 2)
        self.assertEqual(response_json["result"][0]["name"], "user signed up")
        self.assertEqual(response_json["result"][0]["count"], 1)
        self.assertEqual(response_json["result"][1]["name"], "user did things")
        self.assertEqual(response_json["result"][1]["count"], 1)
    
    def test_insight_funnels_basic_get(self):