Adds hoverstates for collapsed Issue/Merge Request sidebar

This MR is part 1/2 of https://gitlab.com/gitlab-org/gitlab-ce/issues/25011

## What does this MR do?

Adds hoverstates for collapsed Issue/Merge Request sidebar

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


## Why was this MR needed?

We needed hover states on the issue/merge request sidebar

## Screenshots (if relevant)

![2016-11-27_23.39.28](/uploads/adb83f5fa291cc8f89f1a7d7e2577546/2016-11-27_23.39.28.gif)![2016-11-27_23.39.13](/uploads/7018e4272da26cb06efb7c6fe4eae413/2016-11-27_23.39.13.gif)

![2016-11-29_16.05.11](/uploads/fcf9a674aa24cf0e7349a7e439b024fe/2016-11-29_16.05.11.gif)

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #25011

See merge request !7777