Auto expand collapsed diffs when in single diff file mode

What does this MR do?

Auto expands collapsed diffs when in single file diff mode. Instead of loading this data from the backend with the initial request, we follow the same behavior as before except that when in single diff file mode we override the collapsed state to make sure we always send an ajax request to get the diff content when viewing a collapsed diff.

To make sure that this change doesn't break anything it is behind a feature flag auto_expand_collapsed_diffs 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

Closes #230511 (closed)