Merge branch '25835-alert-wrapper-creates-extra-margin' into 'master'

Apply margin on alert banners only when there is one or more alerts

## What does this MR do?

Fixes extra margin added to all pages by !8151

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

## Why was this MR needed?

## Screenshots (if relevant)

![screenshot](https://gitlab.com/gitlab-org/gitlab-ce/uploads/f8804454e82a61246d10fa026605fbb3/Screen_Shot_2016-12-19_at_2.04.42_PM.png)

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) 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 [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] 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 it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #25835

See merge request !8209

app/assets/stylesheets/framework/layout.scss

@@ -33,10 +33,12 @@ body {
 }
 
 .alert-wrapper {
-  margin-bottom: $gl-padding;
-
   .alert {
     margin-bottom: 0;
+
+    &:last-child {
+      margin-bottom: $gl-padding;
+    }
   }
 
   /* Stripe the background colors so that adjacent alert-warnings are distinct from one another */