## 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 = self.client.post(
            f"/api/projects/{self.team.id}/insights",
            data={
                "derived_name": "pageview unique users",
                "filters": {
                    "events": [{"id": "$pageview"}],
                    "properties": [{"key": "$browser", "value": "Mac OS X"}],
                    "date_from": "-90d",
                },
            },
        )
        self.assertEqual(response.status_code, status.HTTP_201_CREATED)
        response_data = response.json()
        self.assertEqual(response_data["derived_name"], "pageview unique users")
        self.assert_insight_activity(
            response_data["id"],
            [
                {
                    "user": {"first_name": "", "email": "user1@posthog.com",},
                    "activity": "created",
                    "created_at": "2012-01-14T03:21:34Z",
                    "scope": "Insight",
                    "item_id": str(response_data["id"]),
                    "detail": {
                        "changes": None,
                        "merge": None,
                        "name": "pageview unique users",