Fixes window.opener bug

Adds `noreferrer` value to rel attribute for external links

REF: https://gitlab.com/gitlab-org/gitlab-ce/issues/15331

See merge request !1953

Fix cross-project label ref used with invalid project

When cross-project label reference was used with invalid project reference, `nil` was assigned to project variable which later caused an exception.

Closes #15168

See merge request !3811

Ci::Commit becomes a Pipeline object

1. Ci::Commit receives context: ref, :tag.
1. One Ci::Commit describes a one Pipeline
1. Pipeline is created from `.gitlab-ci.yml`
1. Pipeline is a ordered group of builds
1. We test MR against Pipeline
1. Pipelines have a separate view (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3703)
1. Pipeline can be triggered from UI (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3703)
1. Later we change `Trigger -> TriggerRequest -> Build` to `Trigger -> Pipeline` (future)
1. We add a Pipeline Hook that will be triggered on Pipeline status change  (future)
1. 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.

See merge request !3653

Closes #15168

Merely setting the "action" tag will only result in the transaction
itself containing a value for this tag. To ensure other metrics also
contain this tag we must set the action using Transaction#action=
instead.

# Conflicts:
#	db/schema.rb

Make before_script and after_script overridable

This is makes it possible to overwrite the before_script and after_script at job level.

This is continuation of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3771


See merge request !3772

Implement after_script which allows to do cleanups as part of the build process

This implements `after_script` in global context.

The `after_script` will be executed always after the job, even if the job were canceled.

This requires changes on Runner side that will be implemented in 1.2.

cc @tmaczukin @grzesiek 


See merge request !3771