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:

After:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

cc: @jschatz1