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 |
| --------- | ------ |
|  |  |
Signout (for centering comparison):
## 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
1fed97530a