Resolve "Admin/Overview/Users mobile view"

## What does this MR do?

Fixes mobile layout issues on `/admin/users` page.

* Fixes control element placement when long usernames or email addresses overflow their container
* Fixes condensed dropdown menu (issue caused by acde2e30)

Before:
![Screen_Shot_2016-09-28_at_3.29.11_PM](/uploads/82cd27c367653beb29425065685274dd/Screen_Shot_2016-09-28_at_3.29.11_PM.png)

![Screen_Shot_2016-09-29_at_23.36.40](/uploads/e739638506388be9c5a0709e5f22b298/Screen_Shot_2016-09-29_at_23.36.40.png)

After:
![Screen_Shot_2016-10-24_at_11.55.32_PM](/uploads/ab6e49da85688d9fd2abc49de238a5bf/Screen_Shot_2016-10-24_at_11.55.32_PM.png)

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

Ensure css changes don't have any side effects elsewhere in GitLab.  (I don't think they would...)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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 it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #22746

See merge request !7087