## 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
self.assertEqual(response.status_code, status.HTTP_201_CREATED)
        
        response = self.client.post(
            f"/api/projects/{self.team.id}/dashboards", {"name": "another", "use_template": "DEFAULT_APP",},
        )
        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
        
        response = self.client.post(
            f"/api/projects/{self.team.id}/dashboards", {"name": "another", "use_dashboard": existing_dashboard.id,},
        )
        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
        
        response = self.client.post(
            f"/api/projects/{self.team.id}/dashboards",
            {"name": "another", "use_template": "", "use_dashboard": existing_dashboard.id,},
        )
        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
        
        response = self.client.post(f"/api/projects/{self.team.id}/dashboards", {"name": "another",},)
        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
    def test_dashboard_creation_mode(self):
        
        response = self.client.post(
            f"/api/projects/{self.team.id}/dashboards/", {"name": "another", "use_template": "DEFAULT_APP"}
        )
        self.assertEqual(response.json()["creation_mode"], "template")
        
        existing_dashboard = Dashboard.objects.create(team=self.team, name="existing dashboard", created_by=self.user)
        response = self.client.post(
            f"/api/projects/{self.team.id}/dashboards/", {"name": "another", "use_dashboard": existing_dashboard.id}