Ci::Commit becomes a Pipeline object

1. Ci::Commit receives context: ref, :tag.
2. One Ci::Commit describes a one Pipeline
3. Pipeline is created from .gitlab-ci.yml
4. Pipeline is a ordered group of builds
5. We test MR against Pipeline
6. Pipelines have a separate view (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3703)
7. Pipeline can be triggered from UI (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3703)
8. Later we change Trigger -> TriggerRequest -> Build to Trigger -> Pipeline (future)
9. We add a Pipeline Hook that will be triggered on Pipeline status change (future)
10. We extend notifications to use Pipeline Hook to send summary on pipeline changes (future)

After merging that I'll prepare a separate MR that will unify naming, database columns, table names:

Ci::Commit -> Pipeline
Ci::Build -> Build
CommitStatus -> Job
GenericCommitStatus -> ExternalJob

ci_commits -> pipelines
ci_builds -> jobs

This MR implements first 5 points.

This is made to solve this issue https://gitlab.com/gitlab-org/gitlab-ce/issues/14149.