Change default source link template for alertmanager (#5005)

# What this PR does

Changing source link default template to the link to the first alert in
the group `alerts[0].generatorURL`. `externalURL` as a backlink to the
Alertmanager, is too general

## Which issue(s) this PR closes

Related to [issue link here]

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## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

engine/config_integrations/alertmanager.py

@@ -13,7 +13,7 @@
 
 
 # Behaviour
-source_link = "{{ payload.externalURL }}"
+source_link = "{{ payload.alerts[0].generatorURL }}"
 
 grouping_id = "{{ payload.groupKey }}"
 

engine/config_integrations/grafana_alerting.py

@@ -15,7 +15,7 @@
 
 
 # Behaviour
-source_link = "{{ payload.externalURL }}"
+source_link = "{{ payload.alerts[0].generatorURL }}"
 
 grouping_id = "{{ payload.groupKey }}"