Supports four different event types all bundled under the "note" event type:
- comments on a commit
- comments on an issue
- comments on a merge request
- comments on a code snippet
1) Adds a DB migration for all services to toggle on push, issue, and merge events.
2) Upon an issue or merge request event, fire service hooks.
3) Slack service supports custom messages for each of these events. Other services
not supported at the moment.
4) Label merge request hooks with their corresponding actions.
