Allow SlackService while migrating
- Make the existing migration converting SlackService -> SlackNotificationService a no-op.
- Allow SlackService as an alternative class name for
SlackNotificationService. This also means that
#to_param
returns the same thing for both classes, so the UI will work in either case. - Create a post-deployment migration to update the existing service names in batches.
In a future release, we can remove the dummy SlackService class and migrate any other data from SlackService -> SlackNotificationService.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/25843.