* Add single receiver method
* Add receiver permissions
* Add single/multi GET endpoints for receivers
* Remove stable tag from time intervals
See end of PR description here: https://github.com/grafana/grafana/pull/81672
* declare new API and models GettableTimeIntervals, PostableTimeIntervals
* add new actions alert.notifications.time-intervals:read and alert.notifications.time-intervals:write.
* update existing alerting roles with the read action. Add to all alerting roles.
* add integration tests