Add migration to execute SelfMonitoring::Project::CreateService

What does this MR do?

This MR adds a migration to create a project for self-monitoring the GitLab instance by executing the SelfMonitoring::Project::CreateService service. The service is copied into the migration to prevent changes to the service from causing migration failures later. There is a migration spec as well, which should also help to prevent code changes from causing migration failures.

The db/post_migrate/20190809072552_set_self_monitoring_project_alerting_token.rb migration is EE only. It generates an alertmanager token for the above project, and stores it in the project_alerting_settings DB table.

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15071

Docs MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31530

Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/56883

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers