Add project scoping to split diff feature flag

What does this MR do?

When the split_mr_diff_view was originally added, it didn't include the @project scope.

This MR adds the project when checking the feature flag.

One notable thing here is that in order to test the feature flag on a slow rollout (e.g., just GitLab), we'll need to add this code. Otherwise, we can only test this feature flag on or off globally for all of .com.

The original source of these changes started in this comment.

Context

A little Slack thread.

The conclusion @kerrizor and I came to is that when we check this feature flag in the Diff File Entity, it will always return false, so it's not currently possible to test the feature in production without turning it on globally for all of .com.

Screenshots

N/A (~backstage)

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