## 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/models/insight.py
last_modified_by: models.ForeignKey = models.ForeignKey(
        "User", on_delete=models.SET_NULL, null=True, blank=True, related_name="modified_insights",
    )
    
    dashboard: models.ForeignKey = models.ForeignKey(
        "Dashboard", related_name="items", on_delete=models.CASCADE, null=True, blank=True,
    )
    
    layouts: models.JSONField = models.JSONField(default=dict)
    
    color: models.CharField = models.CharField(max_length=400, null=True, blank=True)
    
    dive_dashboard: models.ForeignKey = models.ForeignKey("Dashboard", on_delete=models.SET_NULL, null=True, blank=True)
    
    updated_at: models.DateTimeField = models.DateTimeField(auto_now=True)
    
    type: models.CharField = deprecate_field(models.CharField(max_length=400, null=True, blank=True))
    
    funnel: models.IntegerField = deprecate_field(models.IntegerField(null=True, blank=True))
    
    deprecated_tags: ArrayField = deprecate_field(
        ArrayField(models.CharField(max_length=32), blank=True, default=list), return_instead=[],
    )
    
    tags: ArrayField = deprecate_field(
        ArrayField(models.CharField(max_length=32), blank=True, default=None), return_instead=[],
    )
    
    MATERIAL_INSIGHT_FIELDS = {"name", "description", "filters"}
    class Meta: