## 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_insight.py
alt_user = User.objects.create_and_join(self.organization, "team2@posthog.com", None)
        self_user_basic_serialized = {
            "id": self.user.id,
            "uuid": str(self.user.uuid),
            "distinct_id": self.user.distinct_id,
            "first_name": self.user.first_name,
            "email": self.user.email,
        }
        alt_user_basic_serialized = {
            "id": alt_user.id,
            "uuid": str(alt_user.uuid),
            "distinct_id": alt_user.distinct_id,
            "first_name": alt_user.first_name,
            "email": alt_user.email,
        }
        
        
        with freeze_time("2021-08-23T12:00:00Z"):
            response_1 = self.client.post(f"/api/projects/{self.team.id}/insights/")
            self.assertEqual(response_1.status_code, status.HTTP_201_CREATED)
            self.assertDictContainsSubset(
                {