Expand Slack Audit Log integration to include events that are not specific to an environment

### Is your feature request related to a problem?

The Slack integration does not capture project-level changes as it is targeted per environment

### Describe the solution you'd like.

A slack integration which matches the audit log for a project 1:1

### Additional context

We currently have a number of integrations built around our Audit Log (Webhook, Datadog, Dynatrace, Slack). We want to standardise these from a UX and code architecture perspective.

### Acceptance criteria

- The Slack integration option is available in the Organisations integrations page.
- The Slack integration produces non-repetitive output in both project or environment setups, e.g. the "feature created" notification isn't repeated for each environment.

There is a dogfooding channel: #slack-integration-dogfood

Trace the existing Slack integration and the Audit Log integrations, including the Organisations integrations page. Compare project-level and environment-level setup and notification output, then use the stated acceptance criteria to verify that the option is available and events are not repeated across environments. The #slack-integration-dogfood channel provides additional context.

Written by the indexing model from the issue text.