Expand diff to entire file

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9870

What does this MR do?

Allows users to toggle the diff between the full file & just the changes.

Screen_Recording_2019-02-20_at_08.55.03

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/19054

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Tested in all supported browsers
• 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.
• Security reports checked/validated by reviewer