Resolve "Change admin navigation to match new UI"

## What does this MR do?
Moves admin navigation to layout nav to match Project, Groups, and Profile navigation

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

## Why was this MR needed?

## What are the relevant issue numbers?
Closes #18338

## Screenshots (if relevant)
![Screen_Shot_2016-06-14_at_10.26.40_AM](/uploads/f0b8c8b259da16d929be2b36e8eeafb8/Screen_Shot_2016-06-14_at_10.26.40_AM.png)
![Screen_Shot_2016-06-14_at_10.26.44_AM](/uploads/07d9ece2063dfcfec1f0f2647d8ee782/Screen_Shot_2016-06-14_at_10.26.44_AM.png)
## Does this MR meet the acceptance criteria?

- [ ] [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
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] 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 you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4650