Introducing label routing

New

• Label routing: map labels to Slack channels per repo, drag rules to set priority when multiple match, and turn on Prevent default posting if you only want labelled PRs/MRs in Slack.
• Docs: new Routing section at /docs/routing - label routing guide included + a screenshot because why not.

Fixed

• Comment webhooks on unmapped repos stop retrying forever and drop cleanly instead of haunting the queue.