WIP: Add change diff to file history view

What does this MR do?

This MR adds diff view to the file history view page.

Todo

• add backend specs
  New specs should be added for this new feature including rspec/feature ones. There are already some specs for other pages that test diff files functionality. I didn't read those spec from the top to the bottom, but I think there's a room for some DRY techniques (move tests to shared_contexts, shared_examples, etc., as I did before for GitLab Edge team). Usually I add specs in the end of the work.
• fix rubocop offenses
  It requires some refactoring, but first the changes of the backend part should be reviewed and accepted just to prevent useless refactoring work.
• fix codeclimate degradation (refactor JS code)
  There are some changes made by me related to javascript. These changes should be DRYed out. This will make codeclimate green again.
• fix line numbers of diffs
• fix dots of diffs
• fix loading indicator
  There's a weird bug related to loading content on scroll. On scroll additional content gets loaded and appended. Can't get reproducible steps so far.
• adjust borders and margins
• update design
• fix JS firing events for Toggle diff with offset > 0

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

Why was this MR needed?

Improves usability.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #36220 (moved)