Allow to disable UI updates on dashboards created using the Rest api

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: Some dashboards are provisioned through API so they will be overwritten by an automated process. But currently we can't disable the save button in the UI. **What would you like to be added**: Add `DisableUiUpdate` to the postDashboard api to disable the save button in the dashboard. **Who is this feature for?** This is useful for the grafana operator and Infrastructure as code tools like terraform, pulumi etc.