Create ActiveRecordModel and table for Merge Train feature

What does this MR do?

This merge request adds a database table and ActiveRecord model which is necessary for Merge Train feature.

Merge train basically allows you to enqueue multiple merge requests to the same target branch, and the merges happen sequentially when its pipeline finished correctly or the previous merge request has been merged.

Reference: https://gitlab.com/gitlab-org/gitlab-ee/issues/9186

• CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27879
• EE: Here

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team