## 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
message = "You don't have edit permissions for this dashboard."
    def has_object_permission(self, request: Request, view, dashboard) -> bool:
        if request.method in SAFE_METHODS:
            return True
        return dashboard.can_user_edit(cast(User, request.user).id)
class DashboardSerializer(TaggedItemSerializerMixin, serializers.ModelSerializer):
    items = serializers.SerializerMethodField()
    created_by = UserBasicSerializer(read_only=True)
    use_template = serializers.CharField(write_only=True, allow_blank=True, required=False)
    use_dashboard = serializers.IntegerField(write_only=True, allow_null=True, required=False)
    effective_privilege_level = serializers.SerializerMethodField()
    class Meta: