Merge branch 'AshleyDumaine/gitlab-ce-22494-wide-custom-header-logos-arent-centered' into 'master'

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

CHANGELOG

@@ -2,6 +2,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.13.0 (unreleased)
   - Use gitlab-shell v3.6.2 (GIT TRACE logging)
+  - Fix centering of custom header logos (Ashley Dumaine)
   - AbstractReferenceFilter caches project_refs on RequestStore when active
   - Speed-up group milestones show page
   - Log LDAP lookup errors and don't swallow unrelated exceptions. !6103 (Markus Koller)

app/assets/stylesheets/framework/header.scss

@@ -112,11 +112,15 @@ header {
     .header-logo {
       position: absolute;
       left: 50%;
-      margin-left: -18px;
       top: 7px;
       transition-duration: .3s;
       z-index: 999;
 
+      #logo {
+        position: relative;
+        left: -50%;
+      }
+
       svg, img {
         height: 36px;
       }
@@ -126,8 +130,12 @@ header {
       }
 
       @media (max-width: $screen-xs-max) {
-        right: 25px;
+        right: 20px;
         left: auto;
+
+        #logo {
+          left: auto;
+        }
       }
     }