templates: add (org-aware) AlertingListURL to ExternalData and fix Slack receiver

[macabu]

Some of the receivers use the /alerting/list for grouping alerts when their generator URLs aren't the same.

However these alerting list URLs are not "org-aware".

Instead of passing an orgId to every receiver constructor, we can enrich the template (extended) data with a URL that has the ?orgID=N in case it exists in the common annotations, to be reused by any receiver.

I've also updated the Slack receiver as an example, since it fixes #69248. If you approve of the approach, I can update the other receivers.