## 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_names = ["a dashboard", "b dashboard"]
        for dashboard_name in dashboard_names:
            self.client.post(f"/api/projects/{self.team.id}/dashboards/", {"name": dashboard_name})
        response = self.client.get(f"/api/projects/{self.team.id}/dashboards/")
        self.assertEqual(response.status_code, status.HTTP_200_OK)
        response_data = response.json()
        self.assertEqual([dashboard["name"] for dashboard in response_data["results"]], dashboard_names)
    @snapshot_postgres_queries
    def test_retrieve_dashboard_list_query_count_does_not_increase_with_the_dashboard_count(self):
        self.client.post(f"/api/projects/{self.team.id}/dashboards/", {"name": "a dashboard"})
        
        start_query_count = len(connection.queries)
        self.client.get(f"/api/projects/{self.team.id}/dashboards/")
        expected_query_count = len(connection.queries) - start_query_count
        self.client.post(f"/api/projects/{self.team.id}/dashboards/", {"name": "b dashboard"})
        self.client.post(f"/api/projects/{self.team.id}/dashboards/", {"name": "c dashboard"})
        
        with self.assertNumQueries(expected_query_count):
            self.client.get(f"/api/projects/{self.team.id}/dashboards/")
    @snapshot_postgres_queries
    def test_retrieve_dashboard(self):