Follow-up from "Moves domain setting to Clusters page"

The following discussions from !24580 (merged) should be addressed:

• @grzesiek started a discussion:

  We should never use real code in migration specs. If it happens that this class we will be extended with ActiveRecord callbacks depending on a NEW column (that does not exist in this database schema version), this .find will raise an exception.

• @grzesiek started a discussion:

  Same remark about using code from app/ in migration spec here.

• @grzesiek started a discussion: (+1 comment)

  We should never mention milestones in the comments. Milestones get changed frequently, and we eventually end up with false promise in the code. See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24998

• @grzesiek started a discussion:

  I advise to remove milestone here, to avoid making false promises.