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.
When a webhook for issues is triggered, it should also return the
resource URL, and the action that was performed (ie: Was it reopened,
updated, opened or closed)