Refine pipeline stages

## What does this MR do?

Introduces a concept of `Ci::Stage` to make it easier to have detailed statuses.

## Why was this MR needed?

This is needed to simplify the handling of `Ci::Statuses` and make the `Stage` actual concept in code:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7889

See merge request !7927