Added option to view diffs file-by-file

What does this MR do?

This was originally a community contribution, see !22427 (closed)

This adds an option for users to view diffs file-by-file, instead of in one big list of files. This can be helpful for some teams, but especially useful if teams often review large merge requests where the performance of the page can suffer.

The option to enable this feature is inside of the user preferences and behind a feature flag (named view_diffs_file_by_file) for now.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

#222790 (closed) #29145 (closed)