Add an API route to delete incidents

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: During the setup process of Grafana incident, we performed many tests. Now real production incidents are mixed with test incidents. Having a way to delete incidents will ensure we keep our history clean, especially during audits and inspections **What would you like to be added**: An API route that allows deleting an incident entirely **Who is this feature for?** Instance administrators, or users with specific permissions.