## 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
_, response_data = self._create_insight(data={"name": "not on a dashboard"})
        self.assertEqual(
            response_data["effective_restriction_level"], Dashboard.RestrictionLevel.EVERYONE_IN_PROJECT_CAN_EDIT
        )
        self.assertEqual(response_data["effective_privilege_level"], Dashboard.PrivilegeLevel.CAN_EDIT)
    def test_an_insight_on_unrestricted_dashboard_has_no_restrictions(self):
        dashboard: Dashboard = Dashboard.objects.create(team=self.team)
        _, response_data = self._create_insight(
            data={"name": "on an unrestricted dashboard", "dashboards": [dashboard.pk]}
        )
        self.assertEqual(
            response_data["effective_restriction_level"], Dashboard.RestrictionLevel.EVERYONE_IN_PROJECT_CAN_EDIT
        )
        self.assertEqual(response_data["effective_privilege_level"], Dashboard.PrivilegeLevel.CAN_EDIT)
    def test_an_insight_on_restricted_dashboard_has_restrictions_cannot_edit_without_explicit_privilege(self):