Check for pipelines when setting page_gutter_class.

## What does this MR do?

Fixes regression issue related to the MR window sliding behind the right sidebar on page refresh. 

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

I'm getting stuck thinking about if/how to test this. Or maybe it just deserves a note somewhere about remembering to update `page_gutter_class` when a new route is added?

## Why was this MR needed?

This regression prevented users from using many features of the MR view.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/21121

## Screenshots (if relevant)

Before: 
![Screen_Shot_2016-08-23_at_11.08.04_AM](/uploads/bad1a83569022a7fc699a978c5abef62/Screen_Shot_2016-08-23_at_11.08.04_AM.png)

After: 
![Screen_Shot_2016-08-23_at_11.07.46_AM](/uploads/6cbcb2f849c68dfc93d8a4b53089b06e/Screen_Shot_2016-08-23_at_11.07.46_AM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [ ] Added for this feature/bug
  - [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)

cc: @jschatz1 

See merge request !5952