Show expand all button on merge request diffs

What does this MR do?

Shows the Expand all button on merge request diffs when at least 1 file is collapsed, instead of all of them being collapsed.

Screen_Recording_2018-11-07_at_13.03.55

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/51737

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.