* Alerting: PoC - default template in k8s templategroup API
* Move code to grafana/alerting
* Move validation to admission validation abstraction
* Testing
* Linting
* Workspace update
* Update grafana/alerting hash to the merged commit
* More user-friendly name and remove need for CREATE/UPDATE validation
Use __default__ for k8s object name and UID so that we don't need to restrict
CREATE/UPDATE for similarly named user-owned templates.
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