## Bug description

Support refreshing all dashboard items. This feature was temporarily removed because it wasn't working properly)

## Expected behavior
- All dashboard items should be properly refreshed and the `items` object in the `dashboardLogic` should contain the latest information.
- From user feedback. I expect items to be automatically refreshed every hour or so (e.g. if I'm a marketer that looks at graphs continuously during the day, or have a screen dedicated just to metrics).


## How to reproduce

1. Create a dashboard with multiple items
2. Attempt to refresh all and check the last refresh timestamp for each item (remember to uncomment relevant code)

## Environment

- [X] PostHog Cloud
- [X] self-hosted PostHog, version/commit: _please provide_

## Additional context
Follow-up to #3363

## Technical context

When refreshing all dashboard items (via `refreshAllDashboardItems()`) not all items are consistently refreshed all the time. Tested for instance with a dashboard with a couple of mixed (different type) graphs. Some items are still kept on "last refreshed 10 minutes ago".

In addition to the above, as originally reported on #3608, when calling `refreshAllDashboardItems` we update each item in their own scope, which means that the general `items` object in the upper scope logic (`dashboardLogic.js`) doesn't get updated. Not sure if we should propagate the change upwards, re-fetch everything, or just do a general refresh.

The UX problem this is causing is that the "Last refreshed" label now introduced in the dashboards gets outdated because `items` doesn't get updated. Having this object with outdated information also leads to other confusing UX issues.


#### *Thank you* for your bug report – we love squashing them!

posthog/api/dashboard.py
validated_data.pop("use_template", None)  
        if validated_data.get("is_shared") and not instance.share_token:
            instance.share_token = secrets.token_urlsafe(22)
        instance = super().update(instance, validated_data)
        if "request" in self.context:
            posthoganalytics.capture(
                self.context["request"].user.distinct_id, "dashboard updated", instance.get_analytics_metadata()
            )
        return instance
    def get_items(self, dashboard: Dashboard):
        if self.context["view"].action == "list":
            return None
        items = dashboard.items.filter(deleted=False).order_by("order").all()
        self.context.update({"dashboard": dashboard})
        return DashboardItemSerializer(items, many=True, context=self.context).data
class DashboardsViewSet(StructuredViewSetMixin, viewsets.ModelViewSet):
    legacy_team_compatibility = True  
    queryset = Dashboard.objects.all()
    serializer_class = DashboardSerializer
    authentication_classes = [
        PublicTokenAuthentication,
        PersonalAPIKeyAuthentication,
        authentication.SessionAuthentication,
        authentication.BasicAuthentication,
    ]
    
    permission_classes = []  
    def get_queryset(self) -> QuerySet: