Mixed (firing/resolved) slack notifications
Mixed (firing/resolved) slack notifications

This commit fixes a problem where resolved alerts are surpressed by still firing alerts.
The subject line now carries both in sections in that case:
e.g.
```
*[CRITICAL - 2]* *[RESOLVED - 1]* *[EU-DE-1]* SomeSillyAlert - Something silly happend
:fire: alert1
:fire: alert2
:white_check_mark: recovered alert
```

sapcc/helm-charts,sapcc/helm-charts,sapcc/helm-charts,sapcc/helm-charts