## 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
_, other_team, _ = User.objects.bootstrap("X", "y@x.com", None)
        Dashboard.objects.create(team=other_team, share_token="testtoken", name="public dashboard", is_shared=False)
        
        response = self.client.get(f"/api/shared_dashboards/testtoken")
        self.assertEqual(response.status_code, status.HTTP_404_NOT_FOUND)
    def test_shared_dashboard(self):
        self.client.logout()
        Dashboard.objects.create(
            team=self.team, share_token="testtoken", name="public dashboard", is_shared=True,
        )
        response = self.client.get("/shared_dashboard/testtoken")
        self.assertEqual(response.status_code, status.HTTP_200_OK)
    def test_share_dashboard(self):
        dashboard = Dashboard.objects.create(team=self.team, name="dashboard")
        response = self.client.patch(
            f"/api/projects/{self.team.id}/dashboards/%s/" % dashboard.pk, {"name": "dashboard 2", "is_shared": True},
        )
        self.assertEqual(response.status_code, status.HTTP_200_OK)
        dashboard = Dashboard.objects.get(pk=dashboard.pk)
        self.assertIsNotNone(dashboard.share_token)
    def test_return_cached_results_bleh(self):