Refactor code and remove `ci_build.stage` column

The following discussions from gitlab-ce!14724 should be addressed:

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

  This would be another place where we depend on ci_builds.stage column, that we actually want to remove completely in the future. This means that we would actually need to have a separate mechanism for creating jobs outside of CreatePipelineService 🤔 But maybe this is enough for now?

• @ayufan started a discussion: (+2 comments)

  We should add validates :stage_id, presence: true, unless: :importing?

• @grzesiek started a discussion:

  This MR is going to close gitlab-ce#37979 but when we decide to address remaining discussions and refactor it we will need to resolve it once again.