Create an alerter that can output to Slack
- Requires a Slack API key with write permissions to
#public_merge_requests_referencing_confidential_issue
- The service should have a test mode so that local & build tests don't spam Slack
- Should gracefully retry if Slack returns an error
- Should raise a noisy error if graceful retries fail
The MVP message should include:
- A link to the MR
- The MR's title
- The Issue's title?