feat(alerting): Add emoji to Threema alert notifications
feat(alerting): Add emoji to Threema alert notifications

This commit prepends emoji to Threema alert notifications to make it
easier to discern various notification types (e.g. alert, no data, ok).

grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana