Merge branch 'center-pipeline-stage-columns' into 'master'

Center build stage columns in pipeline overview

## What does this MR do?
Centers the build stage table content and headers in the [pipeline view](https://gitlab.com/gitlab-org/gitlab-ce/pipelines)

## Are there points in the code the reviewer needs to double check?
Need to check whether it is okay to remove the following ([commit that added this property](https://gitlab.com/gitlab-org/gitlab-ce/commit/6130376ad6673facb729b78878f9156a29948a5b))
```
.light {
    width: 3px;
}
```

## Why was this MR needed?
Improve UI

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-31_at_12.46.43_PM](/uploads/cc4973d2155b59b32ec6fde714fbc26d/Screen_Shot_2016-08-31_at_12.46.43_PM.png)

After:
![Screen_Shot_2016-08-31_at_12.46.28_PM](/uploads/561d7b3973398c827d88870b38136c7d/Screen_Shot_2016-08-31_at_12.46.28_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
- [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 #21708

See merge request !6135

CHANGELOG

@@ -14,6 +14,7 @@ v 8.12.0 (unreleased)
   - Expose `sha` and `merge_commit_sha` in merge request API (Ben Boeckel)
   - Set path for all JavaScript cookies to honor GitLab's subdirectory setting !5627 (Mike Greiling)
   - Fix bug where pagination is still displayed despite all todos marked as done (ClemMakesApps)
+  - Center build stage columns in pipeline overview (ClemMakesApps)
   - Shorten task status phrase (ClemMakesApps)
   - Add hover color to emoji icon (ClemMakesApps)
   - Fix branches page dropdown sort alignment (ClemMakesApps)

app/assets/stylesheets/pages/pipelines.scss

@@ -2,6 +2,7 @@
   .stage {
     max-width: 90px;
     width: 90px;
+    text-align: center;
     overflow: hidden;
     text-overflow: ellipsis;
     white-space: nowrap;
@@ -146,6 +147,7 @@
   }
 
   .stage-cell {
+    text-align: center;
 
     svg {
       height: 18px;
@@ -153,10 +155,6 @@
       vertical-align: middle;
       overflow: visible;
     }
-
-    .light {
-      width: 3px;
-    }
   }
 
   .duration,