* Alerting: Add first Grafana reserved label g_label
g_label holds the title of the folder container the alert. The intention of this label
is to use it as part of the new default notification policy groupBy.
* Add nil check on updateRule labels map
* Disable gocyclo lint on schedule.ruleRoutine
will remove later in a separate refactoring PR to reduce complexity.
* Address doc suggestions
* Update g_folder for rules in folder when folder title changes
* Remove global bus in FolderService
* Modify tests to fit new common g_folder label
* Add changelog entry
* Fix merge conflicts
* Switch GrafanaReservedLabelPrefix from `g_` to `grafana_`
