search: add panel for ranking to frontend dashboard

Created by: stefanhengl

This adds a new panel "ranking" to the frontend dashboard (see screenshot). The data is based on user clicks we track on the search results page.

Test plan: I ran a local instance of Grafana with live production data from Prometheus and inspected the panel visually.

Start a local instance of Grafana. Make sure to run it without Prometheus by removing Prometheus from sg.config.yaml.

sg start monitoring

Forward port 9090 to our production instance of Prometheus

kubectl port-forward -n prod $(kubectl get pod -n prod | grep prometheus | awk '{print $1}') 9090:9090