## 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_dashboard.py
if team_id is None:
            team_id = self.team.id
        response = self.client.post(f"/api/projects/{team_id}/dashboards/", data)
        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
        response_json = response.json()
        return response_json["id"], response_json
    def _get_insight(
        self, insight_id: int, team_id: Optional[int] = None, expected_status: int = status.HTTP_200_OK
    ) -> Dict[str, Any]:
        if team_id is None:
            team_id = self.team.id
        response = self.client.get(f"/api/projects/{team_id}/insights/{insight_id}")
        self.assertEqual(response.status_code, expected_status)
        response_json = response.json()
        return response_json
    def _create_insight(
        self, data: Dict[str, Any], team_id: Optional[int] = None, expected_status: int = status.HTTP_201_CREATED
    ) -> Tuple[int, Dict[str, Any]]:
        if team_id is None:
            team_id = self.team.id
        if "filters" not in data:
            data["filters"] = {"events": [{"id": "$pageview"}]}
        response = self.client.post(f"/api/projects/{team_id}/insights", data=data,)
        self.assertEqual(response.status_code, expected_status)
        response_json = response.json()
        return response_json.get("id", None), response_json