Hide job sidebar on mobile

What does this MR do?

Hides job sidebar on mobile:

With the refactor, I removed some of the js- classes. I did not take in consideration that https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22116 would not be merged for 11.4, which broke the sidebar behavior.

This MR adds the js- classes back and adds specs to make sure this won't happen again.

What are the relevant issue numbers?

Closes #52608 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n.a.] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [n.a.] Conforms to the database guides