Notifying for Alert Rule Creation / Modification

### Problem Statement

I'd like to be able to drop a message in a Slack channel when any alert is created or modified. People tend to hesitate to tune thresholds for alerts they themselves did not create, and my impression is this has to do with people not wanting to be out of alignment with the creator's intentions. My hope is to facilitate more participation in alert tuning by providing more confidence that others can see when alerts are changed and review / comment on changes if desired. I'd want to have this for both standard and metric based alert rules.

### Solution Brainstorm

I'd be happy just to have a webhook for create/modify events on alert definitions so I can implement the functionality myself, but I certainly wouldn't be mad at a direct Slack integration.

### Product Area

Alerts

The issue names no files, tests, or entry points. Start by scoping how standard and metric-based alert rules are created or modified, then review the existing webhook or Slack integration boundaries. Done means create and modification events can notify a Slack channel, either directly or through a webhook.

Written by the indexing model from the issue text.