## 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
self.organization_membership.level = OrganizationMembership.Level.MEMBER
        self.organization_membership.save()
        self.team.access_control = True
        self.team.save()
        response = self.client.get(
            f"/api/projects/{self.team.id}/insights/trend/?events={json.dumps([{'id': '$pageview'}])}"
        )
        self.assertDictEqual(self.permission_denied_response("You don't have access to the project."), response.json())
        self.assertEqual(response.status_code, status.HTTP_403_FORBIDDEN)
    def test_insight_trends_allowed_if_project_private_and_org_member_and_project_member(self):
        self.organization_membership.level = OrganizationMembership.Level.MEMBER
        self.organization_membership.save()
        self.team.access_control = True
        self.team.save()
        self_team_membership = ExplicitTeamMembership.objects.create(
            team=self.team, parent_membership=self.organization_membership, level=ExplicitTeamMembership.Level.MEMBER
        )
        response = self.client.get(
            f"/api/projects/{self.team.id}/insights/trend/?events={json.dumps([{'id': '$pageview'}])}"
        )
        self.assertEqual(response.status_code, status.HTTP_200_OK)
    @patch("posthog.api.insight.capture_exception")
    def test_serializer(self, patch_capture_exception):