Center the header logo

_Originally opened at !6507 by @AshleyDumaine._

- - -

## What does this MR do?

* Centers the header logo using `relative` positioning on the svg or img in the header-logo

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

## Why was this MR needed?

The custom wide header logos aren't centered after logging in as seen in #22494

## Screenshots (if relevant)

| Before | After |
| --------- | ------ |
| ![Screen_Shot_2016-09-24_at_3.35.26_PM](/uploads/576db77e0eb299b06d2a5abf341f058f/Screen_Shot_2016-09-24_at_3.35.26_PM.png) | ![Screen_Shot_2016-09-24_at_3.35.51_PM](/uploads/47237b6c28d438a6f255dffb8b3d39d4/Screen_Shot_2016-09-24_at_3.35.51_PM.png) |

Signout (for centering comparison):
![Screen_Shot_2016-09-24_at_3.35.59_PM](/uploads/4484313ff5841774769c4737584af7fc/Screen_Shot_2016-09-24_at_3.35.59_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [ ] 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 you do - 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 #22494

See merge request !6610