## 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
dashboard = Dashboard.objects.create(name="My Dashboard", team=self.team)
        response = self.client.post(
            f"/api/projects/{self.team.id}/insights",
            data={
                "filters": {
                    "insight": "FUNNELS",
                    "events": [
                        {
                            "id": "$pageview",
                            "math": None,
                            "name": "$pageview",
                            "type": "events",
                            "order": 0,
                            "properties": [],
                            "math_property": None,
                        },
                        {
                            "id": "$rageclick",