## 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/dashboard.py
model = Dashboard
        fields = [
            "id",
            "name",
            "description",
            "pinned",
            "items",
            "created_at",
            "created_by",
            "is_shared",
            "share_token",
            "deleted",
            "creation_mode",
            "use_template",
            "use_dashboard",
            "filters",
            "tags",
            "restriction_level",
            "effective_restriction_level",
            "effective_privilege_level",
        ]
        read_only_fields = [
            "creation_mode",
            "effective_restriction_level",
        ]
    def create(self, validated_data: Dict, *args: Any, **kwargs: Any) -> Dashboard: