Add merge request data to the progress information on Milestones pages.

## What does this MR do?

Adds data about merge requests to the Milestone show view. It previously only showed something like `3 issues: 2 open and 1 closed 20% complete`, now it shows `3 issues: 2 open and 1 closed 2 merge requests: 2 open and 0 merged 20% complete`.

## Are there points in the code the reviewer needs to double check?

I don't think so.

## Why was this MR needed?

The milestone progress % didn't accurately reflect the open/merged MRs.

## What are the relevant issue numbers?

Fixes #13980

## Screenshots (if relevant)

Before:

![Screen_Shot_2016-06-16_at_2.12.57_PM](/uploads/99a044ea918b23fe93cef3305c21086e/Screen_Shot_2016-06-16_at_2.12.57_PM.png)

After:

![Screen_Shot_2016-06-16_at_2.12.38_PM](/uploads/be17e723b9c9bbe33099d1f0e94192d0/Screen_Shot_2016-06-16_at_2.12.38_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

cc: @jschatz1

See merge request !4719