grafana: Enable viewers can edit to ease testing on dashboards.al.org

E.g. when adding new metrics (from monitoring.al.org) or when people
outside the DevOps team want to improve one of the dashboards.

roles/grafana/templates/grafana.ini.j2

@@ -479,7 +479,7 @@ allow_sign_up = false
 ;external_manage_info =
 
 # Viewers can edit/inspect dashboard settings in the browser. But not save the dashboard.
-;viewers_can_edit = false
+viewers_can_edit = true
 
 # Editors can administrate dashboard, folders and teams they create
 ;editors_can_admin = false