## 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, insight_id: Optional[int] = None, team_id: Optional[int] = None, expected_status: int = status.HTTP_200_OK
    ):
        if team_id is None:
            team_id = self.team.id
        if insight_id is None:
            url = f"/api/projects/{team_id}/insights/activity"
        else:
            url = f"/api/projects/{team_id}/insights/{insight_id}/activity"
        activity = self.client.get(url)
        self.assertEqual(activity.status_code, expected_status)
        return activity.json()
    def assert_insight_activity(self, insight_id: Optional[int], expected: List[Dict]):
        activity_response = self._get_insight_activity(insight_id)
        activity: List[Dict] = activity_response["results"]
        self.maxDiff = None
        self.assertEqual(
            activity, expected,
        )