Show deployment status on a MR view

## What are the relevant issue numbers?

Resolves #19571, one in the list of #19992

## Screenshots (if relevant)

external_url = nil
![Screen_Shot_2016-08-02_at_13.57.03](/uploads/20ea1587eea556c7a1acd0ff726a5bfb/Screen_Shot_2016-08-02_at_13.57.03.png)

external_url != nil
![Screen_Shot_2016-08-02_at_13.59.59](/uploads/0094b9ddece3f4bf76c83988840c096d/Screen_Shot_2016-08-02_at_13.59.59.png)

Note, the timings are weird between merging and deploying, that is because I did it in the wrong order.

## Does this MR meet the acceptance criteria?

- [X] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing

See merge request !5622