## 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
dashboard = Dashboard.objects.create(
            team=self.team, name="private dashboard", created_by=self.user, creation_mode="template",
        )
        response = self.client.patch(
            f"/api/projects/{self.team.id}/dashboards/{dashboard.id}",
            {"name": "dashboard new name", "creation_mode": "duplicate", "description": "Internal system metrics.",},
        )
        self.assertEqual(response.status_code, status.HTTP_200_OK)
        response_data = response.json()
        self.assertEqual(response_data["name"], "dashboard new name")
        self.assertEqual(response_data["created_by"]["distinct_id"], self.user.distinct_id)
        self.assertEqual(response_data["creation_mode"], "template")
        self.assertEqual(response_data["description"], "Internal system metrics.")
        self.assertEqual(response_data["restriction_level"], Dashboard.RestrictionLevel.EVERYONE_IN_PROJECT_CAN_EDIT)
        self.assertEqual(
            response_data["effective_privilege_level"], Dashboard.RestrictionLevel.ONLY_COLLABORATORS_CAN_EDIT
        )
        dashboard.refresh_from_db()
        self.assertEqual(dashboard.name, "dashboard new name")
    def test_create_dashboard_item(self):