Grafana: Create optional directories to silence errors when overrides aren't present

Created by: caugustus-sourcegraph

Silences log errors related to missing directories when additional dashboards aren't provided and plugins aren't installed. These empty folders can still be overwritten by mounting volumes (in docker-compose or Kubernetes).

See https://github.com/sourcegraph/sourcegraph/issues/29351 for more context.

The plugins error occurs on both docker-compose and Kubernetes installations, but only displays at startup time. The sg_grafana_additional_dashboards error occurs continuously on Kubernetes installations, but not docker-compose (since we mount a volume here). An alternative solution would be to add a similar sg_grafana_additional_dashboards volume mount for Kubernetes, and ignore the plugins alert, but this seemed like the better option.