Normalize output to use:
- User name instead of username
- Include first line of title in message description
- Link to "Issue #X" instead of "#X"

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.