Milestone tab badges

### What does this MR do?
Adds count badges to the tabs on the Milestone page to mirror the Issue page.

### Are there points in the code the reviewer needs to double check?
CSS: Padding was added to the `.milestone` element to make expired milestones look good.

### Why was this MR needed?
For consistency with other tabs

### What are the relevant issue numbers?
Closes #20114 

### Screenshots (if relevant)

**Before**
<img src="/uploads/1f75853a863a9058ee409916b7ee4057/Screen_Shot_2016-07-22_at_13.22.20.png" width="362">

**After**
<img src="/uploads/8a058290850f571db249dd9b23f5138d/Screen_Shot_2016-07-22_at_13.21.58.png"  width="303">

![Screen_Shot_2016-08-23_at_1.42.56_AM](/uploads/b072dc6bdf505b2fce46660beaf0eae7/Screen_Shot_2016-08-23_at_1.42.56_AM.png)

![Screen_Shot_2016-08-23_at_1.43.15_AM](/uploads/e7440bf2aa628aa4dba63863083699c0/Screen_Shot_2016-08-23_at_1.43.15_AM.png)

### Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [ ] 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)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5946