## 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
"scope": "Insight",
                        "item_id": str(insight_id),
                        "detail": {
                            "changes": [
                                {
                                    "type": "Insight",
                                    "action": "changed",
                                    "field": "name",
                                    "before": "insight name",
                                    "after": "insight new name",
                                },
                            ],
                            "merge": None,
                            "name": "insight new name",
                            "short_id": short_id,
                        },
                    },
                    {
                        "user": {"first_name": "", "email": "user1@posthog.com"},
                        "activity": "created",
                        "created_at": "2012-01-14T03:21:34Z",
                        "scope": "Insight",
                        "item_id": str(insight_id),
                        "detail": {"changes": None, "merge": None, "name": "insight name", "short_id": short_id},
                    },
                ],
            )
    def test_hard_delete_insight(self):
        with freeze_time("2012-01-14T03:21:34.000Z") as frozen_time:
            insight_id, insight = self._create_insight(